There should be some way to identify the number of places that are available for an event. Both in total, as sometimes it matters whether an event in a small gathering of a few people or a coming together of thousands, and also the number of places that remain available, as sometimes it is useful to know that an event is about to sell-out.

This came up in the context of educational courses as events (see email discussion for the schema-course-extend group, also mentioned in issue #1092); but seems broader than educational courses.

I suggest two properties:
eventCapacity: integer; the total number of individuals that may attend the event.
placesRemaining: integer; the number of places for the event that remain unallocated.

(Maybe the inventoryLevel property of Offer can be used for places remaining, but it would require a change to the definition)