maximumPhysicalAttendeeCapacity

This term is proposed for full integration into Schema.org, pending implementation feedback and adoption from applications and websites.

Thing > Property > maximumPhysicalAttendeeCapacity

The maximum physical attendee capacity of an Event whose eventAttendanceMode is OfflineEventAttendanceMode (or the offline aspects, in the case of a MixedEventAttendanceMode).
Values expected to be one of these types
Integer
Used on these types
Event

Source

https://github.com/schemaorg/schemaorg/issues/1842