I have seen an interesting chess setup in Puzzling network of Stack Exchange; there white queens attack all the squares except where the black knights are located, on the other hand all black knights attack the squares where all the queens locate. There is no single square on the board that is not under attack by pieces.
What is the least number of pieces and squares, provided that no one of them under any kind of threat? Pieces need to be same in each colour, for instance black knights against white queens as shown above, or white pawns against black rooks, etc.
For now, I have found that queens may give the optimal solution, but what is the optimal for other pieces?
18 squares with 2 queens, 22 squares with 2 queens, 21 squares with 2 queens. So, for now optimal least number seems to be (but not, in reality) 20 with 18 squares and 2 queens. 20 is lesser than 64, but here in 20's case, the pieces attack each other, so best case is 23 with 21 squares and 2 queens.
64 can be achieved without any piece, so 64 is the worst-case number. We can change the number of pieces, their squares and themselves in order to find least number for each type of piece. Thanks.