For two weeks after an event, Attendees on its page lets you record who came.
It lists everyone who said they were coming, all ticked. Untick anyone who didn't come and press Save.
Their row then reads didn't come, and your count of who came moves with it. The next time you pick people for an event, that past event shows how many came.
After two weeks the record is kept and can no longer be changed. A cancelled event has no Attendees, because nobody was meant to come.