There is no such thing as a 21 move rule in chess. The 50 move rule is a rule in chess that specifies that a player can claim a draw if no pawn has been moved and no piece has been captured in the last 50 moves. This rule is in place to prevent players from simply moving their pieces back and forth without making any progress, which could result in a long and tedious game.
It is worth noting that the 50 move rule is not the only rule in chess that allows for a draw to be claimed. There are several other circumstances in which a draw can be claimed, such as if the same position has occurred three times, or if one player is unable to make a move due to insufficient material.
In conclusion, there is no 21 move rule in chess. The 50 move rule is the only rule that allows a player to claim a draw based on the number of moves that have been made. This rule is in place to prevent players from simply moving their pieces back and forth without making any progress, and to ensure that the game is fair for both players.