Most pieces involved in checkmate in actual game? I recall seeing a number of chess games where a lot of pieces are involved in the final checkmate, and was curious to know what is the maximum number of pieces of one player A that have been involved in a checkmate of the other player B, where a piece is said to be involved if and only if removing it either causes B's king to be no longer in check or causes one of the squares around B's king to be no longer attacked by A's pieces. It is easy to construct a reachable position where 9 pieces are involved, and to prove that no more than 9 pieces are involved. It is also possible for all 6 types of pieces to be involved. For example:

[White "Checkmate"]
[Black "9 pieces involved"]
[FEN "6B1/7K/2N5/5kP1/2R1P2P/8/6N1/3Q4 w - - 0 1"]


But are there any actual competitive games that ended with such a checkmate (i.e. using 9 pieces)? If not, what is the maximum number ever used in a competitive game? Among these games, what is the maximum number of types of pieces used? I exclude non-competitive games merely to avoid contrived games.

Note: As John Coleman pointed out in the comments, this notion of "involved" is more like "critically involved" because it is possible that a checking piece is covering another piece that would also provide check if the first was removed, or that a piece next to the checkmated king is protected by more than one piece. One could consider a variant notion of "loosely involved" defined as follows: A piece X of A is loosely involved in a checkmate of B if and only if we can remove some pieces of A that attack B's king or a square next to B's king such that it is still a checkmate and X is (critically) involved. I think this should cover such cases, so I am fine with including examples that use this looser definition of "involved".
After going through a list of games with king hunts, I found game at last in which 6 pieces are involved in the mate!

[Title "Alexander Alekhine-NN, Simul 34b, Moedling Austria, 3/24/1936"]
[FEN ""]
[startply "33"]

1. e4 e5 2. d4 f6 3. dxe5 fxe5 4. Qh5+ Ke7 5. Qxe5+ Kf7 6. Bc4+ d5 7. Bxd5+ Kg6 8. Qg3+ Kh5 9. Bf7+ g6 10. h3 Qf6 11. Nf3 Be7 12. Qg4+ Bxg4 13. hxg4+ Kxg4 14. Nh2+ Kh4 15. Nf1+ Kg4 16. Be6+ Qxe6 17. f3#


The Bc1 prevents the Black king from moving to f4 and g5, the e4 pawn covers f5, the f3 pawn is giving mate, the Nf1 covers g3, the g2 pawn protects the mating piece and covers h3, and the Rh1 covers the h3, h4, and h5 squares.



Upon seeing this question, I remembered entry #355 of Tim Krabbe’s Chess Diary, which gives some examples of checkmates involving all four minor pieces. The best and simplest example is this game. 

[Title "Vasily Borisovich Malinin-Viktor Savinov, Leningrad Russia, 1988"]
[FEN ""]
[startply "71"]

1. d4 Nf6 2. c4 c5 3. d5 b5 4. cxb5 a6 5. bxa6 Bxa6 6. Nc3 d6 7. Nf3 g6 8. g3 Bg7 9. h4 O-O 10. h5 Nbd7 11. hxg6 hxg6 12. Bh3 Re8 13.  Qc2 Rb8 14. Be3 Nxd5 15. Qxg6 fxg6 16. Be6+ Kf8 17. Nxd5 Rxb2 18. Ng5 Nf6 19. Nf4 Qa5+ 20. Kf1 Bxe2+ 21. Kg1 Reb8 22. Bf7 Rb1+ 23. Kh2 Ng4+ 24. Kh3 Rxh1+ 25. Rxh1 Nxf2+ 26. Bxf2 Bg4+ 27. Kxg4 Rb4 28. Kf3 Qa3+ 29. Be3 Qa8+ 30. Bd5 Qa5 31. Nfe6+ Kg8 32. Nc7+ e6 33. Bxe6+ Kf8 34. Nh7+ Ke7 35. Bg5+ Bf6 36. Bxf6#


This is easily deduced to be a pure mate, thus raising the known record to four pieces.

Also, to user @John Coleman from the comments, here’s a king hunt, with four pieces, for you. :-)

[Title "Roland Schmaltz-Ronen Har-Zvi, Internet Chess Club, 3/7/2001"]
[FEN ""]
[startply "48"]

1. e4 Nc6 2. Nc3 e5 3. g3 Bc5 4. Bg2 Nf6 5. Nge2 d6 6. h3 Bb6 7. O-O O-O 8. Kh2 Re8 9. f4 exf4 10. gxf4 Ng4+ 11. Kg3 Nf2 12. Rxf2 Qh4+ 13. Kxh4 Bxf2+ 14. Ng3 Re6 15. Kg4 Nd4 16. f5 Rg6+ 17. Kf4 Rxg3 18. Qf1 g5+ 19. fxg6 Ne6+ 20. Kf5 Ng7+ 21. Kf6 Rxg6+ 22. Ke7 Re6+ 23. Kd8 Bb6 24. Nd5 Re8#