Introduction:
WARNING:
I ended up creating a separate site (www.practicejapanese.nl) for my "Practice Japanese Series" of tools. The local versions found here are therefore deprecated (not up to date / maintained).
Latest version on www.practicejapanese.nl
Practice Japanese numbers. V1.0.4 (deprecated)
With this tool you can practice the Japanese number system.
Planned additions:
1. Add an option for answering with hiragana as well.
2. Add option to combine manually entering and using the buttons for an answer.
3. Add option to reset counter.
4. Add option for Kanji. (Partly done)
5. Add option for formal numbers (Kanji). (Partly done)
6. Add option for ages (years old) (uses different words / spelling for some numbers).
7. Add option for number of things (uses different words / spelling for some numbers).
8. Add option for number of people (uses different words / spelling for some numbers).
9. Add option for number of ~ etc..
10. Maybe add option for Katakana.
Known bugs:
1. -
Updates:
V1.0.4
1. Added option to practice formal numbers Kanji to the "Japanese to Arabic numerals" tool.
2. Added maru kanji.

Expand to show older updates... V1.0.3
1. Changed zero to use Katakana instead of Hiragana.
2. Added option to practice Kanji to the "Japanese to Arabic numerals" tool.
3. Added option to mix Rōmaji, Hiragana and Kanji in the "Japanese to Arabic numerals" tool.
V1.0.2
1. Fixed a bug where it only "fixed" the first occurrence of alternative (but correct) names used.
2. Fixed a bug where the "kujuu" part of "hyakujuu" incorrectly flaged the answer as incorrect.
V1.0.0
1. Made some small updates finalising the first full release version.
2. Added a "Delete" button for the "Arabic numerals to Japanese" tool.
3. Added instructions for the tool.
V0.1.2
1. Added counters for current streak and correct / incorrectly answered.
2. Added answer that was given incase of an incorrect answer for "Japanese to Arabic numerals".
3. Added option for "Arabic numerals to Japanese" to enter rōmaji of answers that are correct but are not in the button list (ku iso kyuu, zero iso rei etc.).
4. Added jyuu as a correct spelling for 10 (juu).
5. Added check onchange for "Arabic numerals to Japanese" when answer is entered manually.
V0.1.1
1. Fixed bug where "chi" used the hiragana for "sa".
V0.1.0
1. Fixed bug where the tool stoped working if the new correct answer is the previous entered answer.
2. Fixed bug where entering the same "From" number as the "To" number caused an infinte loop.
3. Fixed bug where entering 9999 as "From" number with a lower "To" number caused issues.
4. Added revealable lists for hiragana 0 - 10 000 000 000 000 000 information.
5. Manually entered answers for the "Arabic numerals to Japanese" can now be entered without spaces between the numbers (nijuu vs ni juu).
V0.0.9
1. Added option for Hiragana.
V0.0.8
1. Fixed incorrect spelling for hyaku & byaku.
V0.0.7
1. Fixed bug where the tool wanted "ichi hyaku" iso "hyaku" as the correct answer etc.
2. Fixed bug where the tool wanted "juu rei" iso "juu" as the correct answer etc.
V0.0.6
1. Added system for Arabic numerals to Rōmaji
V0.0.5
1. Added support for rōmaji 1000 - 9999
2. Added revealable lists for rōmaji 100 - 10 000 000 000 000 000

WARNING:
I ended up creating a separate site (www.practicejapanese.nl) for my "Practice Japanese Series" of tools. The local versions found here are therefore deprecated (not up to date / maintained).

Latest version on www.practicejapanese.nl
Practice Japanese numbers. V1.0.4 (deprecated)
With this tool you can practice the Japanese number system.
Planned additions:
1. Add an option for answering with hiragana as well.
2. Add option to combine manually entering and using the buttons for an answer.
3. Add option to reset counter.
4. Add option for Kanji. (Partly done)
5. Add option for formal numbers (Kanji). (Partly done)
6. Add option for ages (years old) (uses different words / spelling for some numbers).
7. Add option for number of things (uses different words / spelling for some numbers).
8. Add option for number of people (uses different words / spelling for some numbers).
9. Add option for number of ~ etc..
10. Maybe add option for Katakana.
Known bugs:
1. -
Updates:
V1.0.4
1. Added option to practice formal numbers Kanji to the "Japanese to Arabic numerals" tool.
2. Added maru kanji.

Expand to show older updates... V1.0.3
1. Changed zero to use Katakana instead of Hiragana.
2. Added option to practice Kanji to the "Japanese to Arabic numerals" tool.
3. Added option to mix Rōmaji, Hiragana and Kanji in the "Japanese to Arabic numerals" tool.
V1.0.2
1. Fixed a bug where it only "fixed" the first occurrence of alternative (but correct) names used.
2. Fixed a bug where the "kujuu" part of "hyakujuu" incorrectly flaged the answer as incorrect.
V1.0.0
1. Made some small updates finalising the first full release version.
2. Added a "Delete" button for the "Arabic numerals to Japanese" tool.
3. Added instructions for the tool.
V0.1.2
1. Added counters for current streak and correct / incorrectly answered.
2. Added answer that was given incase of an incorrect answer for "Japanese to Arabic numerals".
3. Added option for "Arabic numerals to Japanese" to enter rōmaji of answers that are correct but are not in the button list (ku iso kyuu, zero iso rei etc.).
4. Added jyuu as a correct spelling for 10 (juu).
5. Added check onchange for "Arabic numerals to Japanese" when answer is entered manually.
V0.1.1
1. Fixed bug where "chi" used the hiragana for "sa".
V0.1.0
1. Fixed bug where the tool stoped working if the new correct answer is the previous entered answer.
2. Fixed bug where entering the same "From" number as the "To" number caused an infinte loop.
3. Fixed bug where entering 9999 as "From" number with a lower "To" number caused issues.
4. Added revealable lists for hiragana 0 - 10 000 000 000 000 000 information.
5. Manually entered answers for the "Arabic numerals to Japanese" can now be entered without spaces between the numbers (nijuu vs ni juu).
V0.0.9
1. Added option for Hiragana.
V0.0.8
1. Fixed incorrect spelling for hyaku & byaku.
V0.0.7
1. Fixed bug where the tool wanted "ichi hyaku" iso "hyaku" as the correct answer etc.
2. Fixed bug where the tool wanted "juu rei" iso "juu" as the correct answer etc.
V0.0.6
1. Added system for Arabic numerals to Rōmaji
V0.0.5
1. Added support for rōmaji 1000 - 9999
2. Added revealable lists for rōmaji 100 - 10 000 000 000 000 000

Instructions:
Disclaimer:
Everything stated / used here should be correct. I am however just starting to learn myself and am not a expert in Japanese at all, so if you see any issues, errors and / or discrepancies please contact me at bta.smits@outlook.com.
General:
1. Check "Number system explanation" below on which answers the tool is going to expect.
2. Click on the spoiler button to reveal the information.
3. Click on the downwards arrow to reveal the options for more numbers.
4. Choose which one you would like to practice: "Japanese to Arabic numerals" or "Arabic numerals to Japanese".
Japanese to Arabic numerals:
1. Here the tool will generate a random number between 0 and 9999 and will display it in Japanese.
2. It is up to you to enter the corresponding Arabic ("standard western") numeral.
3. First select the type of numbers the tool should generate (Rōmaji, Hiragana, Kanji etc.).

Expand to show more... 4. Then select the range of numbers between which the tool will generate it's numbers by entering the values in the "from" and "to" boxes.
5. The tool will display a number in Japanese and you can enter the number in the small entry box.
6. After entering the number you can either: 1. press "Tab" 2. press "Enter" 3. click outside the box 4. tab the "check answer" button.
7a. The tool will display if you entered the correct answer and if needed show the correct answer vs what you entered.
7b. The tool will now display the next number for you to answer.
8. Below this you can check your current streak of correctly answered questions and your total correctly and incorrectly answered questions.
Arabic numerals to Japanese:
1. Here the tool will generate a random number between 0 and 9999 and will display it with a Arabic ("standard western") numeral.
2. It is up to you to enter the corresponding Japanese name of this number.
3a. First select if the "answer buttons" are displayed in rōmaji or hiragana.
3b. The answer always needs to be entered in rōmaji however.
4. Then select the range of numbers between which the tool will generate it's numbers by entering the values in the "from" and "to" boxes.
5. The tool will display a number and you can then enter the Japanese name (in rōmaji) in the entry box below.
6a. You can either enter the number manually or by clicking the corresponding buttons below.
6b. These options can not be combined as clicking one of the buttons after having manually entered something will process that answer first.
6c. Entering manually can be done with or without spaces (juu ni vs. juuni).
6d. You can "build up" an answer by using the buttons. For example: build juu ni by first clicking the "juu" and then the "ni" button.
6e. If you made a mistake while using the buttons click the "Delete" button to remove the entered answer.
7a. After entering the number manually you can either: 1. press "Tab" 2. press "Enter" 3. click outside the box 4. tab the "check answer" button.
7b. If you used the buttons you can click the "check answer" button.
8a. The tool will display if you entered the correct answer and if needed show the correct answer vs what you entered.
8b. The tool will now display the next number for you to answer.
9. Below this you can check your current streak of correctly answered questions and your total correctly and incorrectly answered questions.

Disclaimer:
Everything stated / used here should be correct. I am however just starting to learn myself and am not a expert in Japanese at all, so if you see any issues, errors and / or discrepancies please contact me at bta.smits@outlook.com.
General:
1. Check "Number system explanation" below on which answers the tool is going to expect.
2. Click on the spoiler button to reveal the information.
3. Click on the downwards arrow to reveal the options for more numbers.
4. Choose which one you would like to practice: "Japanese to Arabic numerals" or "Arabic numerals to Japanese".
Japanese to Arabic numerals:
1. Here the tool will generate a random number between 0 and 9999 and will display it in Japanese.
2. It is up to you to enter the corresponding Arabic ("standard western") numeral.
3. First select the type of numbers the tool should generate (Rōmaji, Hiragana, Kanji etc.).

Expand to show more... 4. Then select the range of numbers between which the tool will generate it's numbers by entering the values in the "from" and "to" boxes.
5. The tool will display a number in Japanese and you can enter the number in the small entry box.
6. After entering the number you can either: 1. press "Tab" 2. press "Enter" 3. click outside the box 4. tab the "check answer" button.
7a. The tool will display if you entered the correct answer and if needed show the correct answer vs what you entered.
7b. The tool will now display the next number for you to answer.
8. Below this you can check your current streak of correctly answered questions and your total correctly and incorrectly answered questions.
Arabic numerals to Japanese:
1. Here the tool will generate a random number between 0 and 9999 and will display it with a Arabic ("standard western") numeral.
2. It is up to you to enter the corresponding Japanese name of this number.
3a. First select if the "answer buttons" are displayed in rōmaji or hiragana.
3b. The answer always needs to be entered in rōmaji however.
4. Then select the range of numbers between which the tool will generate it's numbers by entering the values in the "from" and "to" boxes.
5. The tool will display a number and you can then enter the Japanese name (in rōmaji) in the entry box below.
6a. You can either enter the number manually or by clicking the corresponding buttons below.
6b. These options can not be combined as clicking one of the buttons after having manually entered something will process that answer first.
6c. Entering manually can be done with or without spaces (juu ni vs. juuni).
6d. You can "build up" an answer by using the buttons. For example: build juu ni by first clicking the "juu" and then the "ni" button.
6e. If you made a mistake while using the buttons click the "Delete" button to remove the entered answer.
7a. After entering the number manually you can either: 1. press "Tab" 2. press "Enter" 3. click outside the box 4. tab the "check answer" button.
7b. If you used the buttons you can click the "check answer" button.
8a. The tool will display if you entered the correct answer and if needed show the correct answer vs what you entered.
8b. The tool will now display the next number for you to answer.
9. Below this you can check your current streak of correctly answered questions and your total correctly and incorrectly answered questions.

Number system explanation:
Note:
Different versions / stylizations of the rōmaji are possible, the ones used on this site are how they are in my textbook.
For example: kyuu vs. kyū vs. kyu- or ni hyaku vs. nihyaku vs. ni-hyaku etc.
Rōmaji 0-10:
SPOILER

Rōmaji 10-19:
SPOILER
Rōmaji 10-90 tens:
SPOILER
Rōmaji 20-29 (20 - 99):
SPOILER
Rōmaji 100-900 hundreds:
SPOILER
Rōmaji 1000-9000 thousands:
SPOILER
Rōmaji 10,000-10,000,000,000,000,000:
SPOILER

Hiragana 0-10:
SPOILER

Hiragana 10-19:
SPOILER
Hiragana 10-90 tens:
SPOILER
Hiragana 20-29 (20 - 99):
SPOILER
Hiragana 100-900 hundreds:
SPOILER
Hiragana 1000-9000 thousands:
SPOILER
Hiragana 10,000-10,000,000,000,000,000:
SPOILER

Kanji 0-10:
SPOILER

COMING SOON

For more info:
Japanese From Zero! (Book used)
Japanese numerals (Wikipedia)
Counting in Japanese (Japanese Language Wiki)
Note:
Different versions / stylizations of the rōmaji are possible, the ones used on this site are how they are in my textbook.
For example: kyuu vs. kyū vs. kyu- or ni hyaku vs. nihyaku vs. ni-hyaku etc.
Rōmaji 0-10:
SPOILER

Rōmaji 10-19:
SPOILER
Rōmaji 10-90 tens:
SPOILER
Rōmaji 20-29 (20 - 99):
SPOILER
Rōmaji 100-900 hundreds:
SPOILER
Rōmaji 1000-9000 thousands:
SPOILER
Rōmaji 10,000-10,000,000,000,000,000:
SPOILER

Hiragana 0-10:
SPOILER

Hiragana 10-19:
SPOILER
Hiragana 10-90 tens:
SPOILER
Hiragana 20-29 (20 - 99):
SPOILER
Hiragana 100-900 hundreds:
SPOILER
Hiragana 1000-9000 thousands:
SPOILER
Hiragana 10,000-10,000,000,000,000,000:
SPOILER

Kanji 0-10:
SPOILER

COMING SOON

For more info:
Japanese From Zero! (Book used)
Japanese numerals (Wikipedia)
Counting in Japanese (Japanese Language Wiki)