i had a 5-8 game stretch with ~20160 selled places, varying entry fees and free bleachers ;)
When i bring that up, someone had a theory that it is dependent on the arena size, don't know if he was right but i cracked this number at the first PO game(if my memory is right i cans earch a screenshot tomorrow), and i build some extra seats meanwhile.(another explanation would be a boost through cup final)