A number part of speech is a numeral or number word.
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.
I'm panicking a bit because there are so many number words in Japanese.
How did you memorize numerals, Koichi?
I wish there was a game that could help me study Japanese numerals.