Several parts combine into one character.

數 (traditional) is a phono-semantic character: 攴 (to tap, a hand holding a stick) for meaning and 婁 (a star name) for sound. From the action of tallying one by one came "to count, a count, number." The shinjitai and simplified reduce the left side to write 数.

So the form splits: traditional (ko/en/es) is 數, shinjitai (ja) and simplified (zh) are 数.

Korean hanja-sound "su": 數學 (suhak, mathematics), 數字 (sutja, digit), 點數 (jeomsu, score), 複數 (boksu, plural), 無數 (musu, countless), 運數 (unsu, fortune / fate). 數 also means "fate / luck" — "the 數 has run out" means one's luck is gone.

Mandarin: shù, 4th tone (number) / shǔ, 3rd tone (to count) / shuò, 4th tone (frequently). 数字 (shùzì, digit), 数学 (shùxué, mathematics), 数据 (shùjù, data), 数一数 (shǔ yī shǔ, to count). A one-character, three-sound case — "to count" is shǔ (3rd), "number" is shù (4th).

Japanese: on-readings スウ (sū) and ス (su), kun かず (kazu) and かぞ.える (kazoeru). 数 (kazu, number), 数学 (sūgaku, mathematics), 数字 (sūji, digit), 人数 (ninzū, number of people), 手数 (tesū, trouble / effort). It is the 数 you use to count pieces of sushi.

Memory aid: a hand with a stick (攴) tallying one by one — number comes from the act of counting.