Wifi and Collision Domains

Given that multiple clients in a single collision domain can impact performance, and given that a WAP functions as a single collision domain, does it follow that more clients on a single WAP will increase the number of collisions that take place, and if so, what performance impact might that realistically have?

I see WAPs that advertise support for 300+ clients. Would supporting, say, even half this number cause performance impacts from collisions? Or is this more theoretical with little real world impact?

Thanks!