You can configure your event's registration to allow students to register to a waitlist once a maximum number of registrations have been made. On the Basic Details tab of the event's configuration, choose "Yes" to the question asking if registration has a maximum number of registrations. Once you enter the max number of registrations allowed, you can activate a waitlist as shown in the below screenshot.
NOTE: Families will not be able to make a payment until you approve the pending registration from your admin dashboard.