What is the highest number of pieces from one side that can be involved in a checkmate? What is the highest number of pieces from one side that can be involved in a checkmate? &quot;Involved in a checkmate&quot; is defined below.
A non-king piece is involved in a checkmate 'if removing it from the checkmate position would change the position to one which is either not checkmate or is illegal'.  The king is involved in a checkmate if it is the only piece of the checkmating side that attacks one of the other king's escape squares.
Here is a solution with all 16 white pieces, all mandatory for checkmate, without promoted units and without needing to protect the white king from checks. If you remove any white piece but the king, the position is still legal but not a checkmate anymore. Moving the wK to a random square (say, h1) would also destroy the mate.

QNr1R1b1/P1BpnP2/3P4/qP4Pr/Rn2kP2/1N4PB/2P2K2/8 w - - 0 1


Last move is Nc6-b8# or d5-d6#.