Place the whole army of White and the black king on the board, such that no piece is attacked or defended. (Obviously you must use double pawns.) That's fairly easy, so tie breaker will be how many black pawns you can additionally smuggle in (again no piece being attacked or defended).
Working example (I placed stuff random):
[FEN "1K6/6NB/p7/3Q4/5R2/2R5/4P3/k3N1B1 w KQkq - 0 1"]
As you can see, no more remaining White piece fits on the board (pawns on 1st/8th are illegal - please only legal positions, otherwise 32 knights would be the way to maximally "fill" a board). You could place a third knight on h8 but first try to get all White 8 pawns aboard...