I can do

>! 5 pieces (2K,3Q):
>! <br> [![enter image description here][1]][1]
>! <br> [FEN "Q2Q2/4k1Q1/8/3K4/8/8/8/8 w - - 0 1"]
>! <br> The mating move would have been a pawn promotion: f7-f8=Q# 

  [1]: https://i.sstatic.net/1ixWF.png