What is the maximum number of en passants possible? Out of curiosity, how many en passants can theoretically be done by both sides and the fastest way to achieve that result?
Every pawn has to take a pawn in an en passant capture, and each pawn taking another pawn will leave the 5th rank. Hence, the total possible number of en passant captures in a game is simply the number of pawns halved, i.e. 8. It can be achieved in the following sequence of moves:

[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

1. h4 a5 2. h5 g5 3. hxg6/ep a4 4. b4 axb3/ep 5. a4 h5 6. a5 b5 7. axb6/ep h4 8. g4 hxg3/ep 9. c4 f5 10. c5 d5 11. cxd6/ep f4 12. e4 fxe3/ep 13. f4 c5 14. f5 e5 15. fxe6/ep c4 16. d4 cxd3/ep


This is the minimum number of moves required for 8 en passant captures, and this is due to:


All moves being pawn moves
Every pawn advancing 2 squares at its first move