Capacity by the 500, not by the seat
Each block adds room for 500 attendees in any single event that month. Attendees receive video and screen share; they do not send any. That keeps the load on your network flat: a 720p stream down is about 1.2 Mbps per viewer, and nothing goes up except Q&A text and reactions.
If an event goes over its blocks, nobody is turned away at the door and the host sees no upgrade screen. The overrun shows in the admin console and we discuss it at the next monthly bill.