I'm not sure exactly what you're asking...
Do you want to know how many interview invites the average applicant rakes in during a given cycle, or how many interview dates each school gives applicants to choose from?
In either case, you really can't generalize. Many applicants are not invited to interview at all, and many applicants interview at 50%+ of their schools. As for the invites themselves, that also varies. Some schools give students have a dozen dates to choose from, while others offer only one.