Head into the editor for the Event Calendar you wish to edit.
Click on the event that you wish to edit. The sidebar will change and present you with a set of new options.
Click the RSVP/Tickets tab at the top of the sidebar. This will present the options for RSVP/Tickets.
Select 'Allow RSVP/Tickets' and 'Limit number of RSVP/Tickets'. You can then choose the exact amount of RSVP's or tickets available for purchase.
In terms of set up, that's it!
When all the available tickets have been purchased, a 'sold out' button will then display and your customers will no longer be able to RSVP or purchase.