Why cap before the need is obvious
Rooms have sizes. Whatever platform you present on, the practical ceiling for a session that stays good โ everyone seen, everything audible, your attention spread wide enough โ is a real number, and the cap is how the registration side of the evening agrees with it. There is also a plainer reason to choose the number deliberately: an uncapped page fills itself to wherever the internet carries the link, and a large registered list with a small room produces apologies, not order.
Capping early is cheap and capping late is not. Decided before the invitations go out, a limit is just a setting. Decided afterwards, every change is a message to people who booked against the old number.