How many knights can you fit on a chess board without any being threatened?
32 knights: all on white or all on black. When a knight moves, it moves to a space of a different color. If they are all on the same color, no collision happens.
20 or hundreds, depending on the size of the pieces and board.
Either you are changing the rules of chess for this puzzle or you are not.
If you are changing the rules of chess, then why not change the way that knights move and change that you can only have 1 piece per square?
If you are not changing the rules, then the most you can get is 2(original)+8(turn each pawn into a knight)) per player