Though there isn't a hard cap, you can't have an enthusiasm higher than 12 or maybe 13 on game day. It decreases exponentially the higher you go up, so if you have 15 or 16 one day it will be down to 12 two days later. If you TIEd your cup championship, and were somehow dominant enough to TIE every single game preceding it that season, you would have the highest enthusiasm you could possibly have in a season the following Saturday. I can't say how much that is, because I don't know of anyone that has or could now feasibly accomplish that, but I still don't think you'd be over 13.
As for the playoffs, you can’t have more than 10 outside of the cup, so you could play PON on the Tuesday of your first game, but you don’t have another game that can build enthusiasm until Saturday, so at best you’d have 10 on the second game of the playoffs, and playing PON in that one would give you an enthusiasm of 12 in the tiebreaker. 13 would be a stretch, though perhaps not impossible, but you’ll never come close to getting enthusiasm at 20+, even right after a game.
I've never had an enthusiasm of higher than 12 myself on a game day, and I TIE virtually every game. There technically isn’t a cap, but most teams won’t play a game with enthusiasm over 12.