Not in front of my computer right now but I’ll leave this here since I had it bookmarked: https://community.hubspot.com/t5/CRM/How-to-create-a-number-of-associated-tickets-property-for/m-p/366849
I don’t see why you couldn’t applyt he same logic to count the number of event attendance records.
While editing the calculated field, you can also apply conditions, so you can count only records matching a certain criteria.