What is the most possible checkmates in one without promoted units in a position? It must all be legal, of course. Promotions are allowed in the game, but the position is not allowed to start with promoted pieces.
After some asking on Matplus a few weeks ago, here are the final records.
The 43 Mates:
Pd2-1, Pf2-2, Ph2,1, Nc3-2, Nh4-2, Qd4-4, Kf7-6, Rg5-12, Be4-13
The 47 Mates:
Pa7-1, Pd7-1, Ph7-1, Ne8-2, Qf5-4, Nd8-8, Be5-13, Rc4-14
I have 35, with the following:
Pg7: 1 (Q or B)
Qf3: 2 - 4 possible queen checks leave the rook at e4 unprotected
The secret to the moves here is:
Double protecting all positions around the king, so that I could move any of those pieces to check without worrying that I was leaving a spot open for the king to move into.
Here is an alternate solution with 47 mates that I found, originally posted here: https://www.chess.com/forum/view/more-puzzles/diams-totally-puzzled-213-page-66-ndash-mate-in-one?page=2
Hmmm... I also just found out at matplus this alternate solution was first found in 1882... oh well! There's nothing new under the sun I guess...