In order to limit an event registration form to only allow registrations up to the event's capacity, this capacity must be declared on the Event Record in the Raiser's Edge. Then the Event Registration Form part in NetCommunity can then be configured to limit registrations to that number. 

STEPS:
1. In the Event Record inside the Raiser's Edge, in the field marked "Capacity," enter the maximun number of registrations you would like to allow. 
2. Save and close the Event Record.  
3. Log into NetCommuntiy and either as you are creating or editing an existing Event Registration Form part, there is a checkbox that must be checked in order to limit this event to it's capacity.