The best solution I could come up with is nine kings. I think kings work best if you don't want to have any square covered more than once since their movement is very simple and short range so have multiple kings on the board won't cover the same squares multiple times as having multiple rooks on the board at the same time would.
In the diagram, let each pawn equal a king.
What would be the smallest group of chess pieces that could cover ALL the bboard?And without having a squate covered by more than a piece?( without sopreposition)