There are four Kanji.
Meaning:
1 | 2
3 | 4
Readings:
1: ichi, hito, hitotsu
2: ni, futa, futatsu
3: san, mi, mittsu
4: shi, yon, yo, yottsu/yotsu

Interesting fact: In Japan 4 is an unlucky number because of the reading "shi", as the Kanji of death has the same reading.