Numeral
にまん
このぶどうは二万円もしました。
These grapes were 20,000 yen!
二万フォロワーおめでとうございます。
Congratulations on twenty thousand followers!
二万ドルかけるよ。
I will bet twenty thousand dollars.
When you have two ten thousands, you have twenty thousand.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.