Hey Rookayah!
There is no 21 move rule for a win, but there IS a 50 move rule and a 3 move repetition rule for a draw. Here is how they work:
50 move rule
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If 50 moves are made by both sides without a pawn move or capture, then one of the players can claim a draw.
3 move repetition
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If the same position is repeated throughout the entirety of the game three times, then one of the players can claim a draw.
I can't recall any other rules involving the number of moves. I hope this helps.
Best,
Nick