Shortest possible game with 18 queens? The selected answer to this question shows a game in which all 16 pawns promote to queens so that there is a total of 18 queens on the board. It takes 80 moves...


  What is the shortest sequence of moves that achieves this goal?


And just to make things interesting, is the minimum number the same if we ask for 20 rooks, bishops, or knights to be present? This is not obvious, because one of the solutions could require extra moves (for instance, to remove a king from a check given by a new queen).
Solution by Friedrich Burchard &amp; Friedrich Hariuc (1976) in 96 Half-moves

[FEN ""]

1.e4 f5 2.e5 Nf6 3.exf6 e5 4.g4 e4 5.Ne2 e3 6.Ng3 e2 7.h4 f4 8.h5 fxg3
9.h6 g5 10.Rh4 gxh4 11.g5 g2 12.g6 Bg7 13.hxg7 g1=Q 14.f4 h3 15.f5 h2
16.b4 a5 17.b5 a4 18.b6 a3 19.Bb2 Ra7 20.bxa7 axb2 21.a4 b5 22.a5 b4
23.a6 b3 24.c4 h1=Q 25.c5 h5 26.c6 Bb7 27.cxb7 c5 28.d4 c4 29.d5 Nc6
30.dxc6 c3 31.c7 c2 32.c8=Q c1=Q 33.b8=Q Qcc7 34.a8=Q d5 35.a7 d4
36.Nc3 dxc3 37.Qa6 c2 38.Qa8b7 c1=Q 39.a8=Q Qhd5 40.gxh8=Q+ Kd7 41.g7
bxa1=Q 42.g8=Q b2 43.f7 b1=Q 44.f8=Q h4 45.f6 h3 46.f7 h2 47.Qfa3 h1=Q
48.f8=Q exf1=Q+


Credit goes to d'alar'cop from  his answer.