There are some difference in client wifi interfaces (STA) and access point wifi interfaces (APs, like you'd find on a good router). For example, some wifi interfaces don't have promiscuous mode, or can't scan while maintaining an active connection, etc.

It's like the difference between softmodems (aka winmodems) and full hardware modems. I know there are some projects that use Raspberry Pis as an AP, and it could do like 10 devices stock and 20 devices with firmware changes. Even a low-end router could handle more clients than that.