In a completed game, click on the game and go to spectators tab, scroll down you can see the attendence even if you haven't attended that game.

I think you can increase it a bit for your level. The maximum amount you may gain in a match from tickets is linked to the number of fans in both teams in the match, meaning you can increase your ticket price if you are playing against top players in your league.

My level is 21 with 4.49m fans, current setting is league ticket 45, cup 50 and CL 50. Friendly with jose can sell for 18...