How many knights can you fit on a chess board without any being threatened?
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