If you're hosting an event with limited space, you can enable a waitlist to ensure that only a certain number of RSVPs can come in at a given time.
At the bottom of your event management page, you can choose to limit the number of guests who can RSVP (including hosts).
When you send out the invite to all your guests if the number of guests attending reaches capacity everyone else who RSVP's yes will be informed the event is full with an option to be added to the waitlist.
If guests who are planning to attend change their RSVP to "Can't Attend" all guests on the waiting list will be informed of the opening. Those who are still interested in attending will be taken off the waitlist on a first-come-first-served basis.