I consulted with a peer who has a better understanding than I, and this is what they had to say: It comes down to the strength of the radio and the bandwidth of the communication more than number of clients. By default, the DHCP server only assigns 150 ips, and the network could only support 252 more devices without configuring the IP network.
We looked at openwrt docs to learn that the default limit is 150, but I very much doubt that 150 simultanous clients could be supported. As I read through all of the resources that are available, but which are not directly related to the MT7620, my general understanding is that too much of this answer lies in variables that are determined by the actual use case. If the client is just pinging a service on the AP, then perhaps 100s may be supported. If the client is requesting access to a streaming web cam, then that number will be substantially lower.