Hello,
The most straightforward way would be to get the total capacity of the event (EventCapacity column) and subtract the total number of registered attendees (Events_Attendee table). I would recommend you to check the following objects in our 
API reference: 
CMS.EventManager.EventAttendeeInfoProviderCMS.EventManager.EventProviderYou can place your code into a custom transformation method as described 
here.
Best regards,
Boris Pocatko