Once you’ve learned the ABCs of Japanese, the kana, it makes sense to learn the 123s as well:

#	Kanji	Meaning	Grade	Kanken Level
1	一	one	1	10
2	二	two	1	10
3	三	three	1	10
4	四	four	1	10
5	五	five	1	10
6	六	six	1	10
7	七	seven	1	10
8	八	eight	1	10
9	九	nine	1	10
10	十	ten	1	10
11	百	hundred	1	10
12	千	thousand	1	10
13	万	ten thousand	2	9

Only thirteen numbers? How much could you count with those? Quite a lot! Just as we combine Arabic numerals to make larger numbers, so we can combine kanji:

Kanji	Reading	Interpretation	Value
十一	じゅういち	10 + 1	11
十二	じゅうに	10 + 2	12
二十	にじゅう	2 × 10	20
二十三	にじゅうさん	2 × 10 + 3	23
八百三十一	はっぴゃくさんじゅういち	(8 × 100) + (3 × 10) + 1	831
百万	ひゃくまん	100 × 10,000	1,000,000
九千九百万	きゅうせんきゅうひゃくまん	(9000 + 900) × 10,000	99,000,000

Try converting a few numbers to Japanese yourself. You can check if you’re right with this tool. Put in a number and it will tell you the correct kanji representation, as well as how to read it.

Now that you know how to combine numbers, you’re ready to count anything you like, right? Five apples, seventeen rabbits, a thousand something else.

Not quite.

In Japanese, when you want to say how many of something there are, you use the number with a suffix called a counter that specifies what kind of thing you’re counting.

For apples, or other small objects, the counter is 個 (こ), so to say five apples, you’d say 五個 (ごこ) or ５個:

箱にはりんごが5個入っている。

There are five apples in the box.

For rabbits (and birds), the counter is 羽 (わ), so for seventeen rabbits, you’d say 十七羽 (じゅうしちわ) or １７羽.

There are lots of different counters for different kinds of objects, so here’s just one more. If you want to count miscellaneous things, or objects that don’t have their own counter, you can use つ. So, for a thousand objects, you could say 千つ (せんつ).

There’s just one more thing to note about Japanese numbers before we wrap up. What if you only need one or two things, rather than a thousand? 一つ or 二つ, right? Right. And how would you read these words? いちつ or につ, right? Wrong! They are read ひとつ and ふたつ, because they use the Japanese reading for the numbers (ひと and ふた) rather than the Chinese reading (イチ and ニ).

Kanji	Reading
一つ	ひとつ
二つ	ふたつ
三つ	みっつ
四つ	よっつ
五つ	いつつ
六つ	むっつ
七つ	ななつ
八つ	やっつ
九つ	ここのつ
十	とお
二十つ	にじゅうつ
二十三つ	にじゅうみっつ

Some other counters use the Japanese readings for numbers, too. And as with other Japanese compounds, sometimes the sounds of the numbers change when they are combined with a counter, e.g. 三本 three long, thin objects is read さんぼん rather than さんほん.

That’s enough about counting! If you’re interested in learning more, Wikipedia has an article with a lengthy list of counters and more details about how to use them and how to read them.