There's a (theoretical) absolute limit of 65535 concurrent connections. When using SNAT or MASQUERADE, that is. As such, the maximum feasible number of devices would be somewhere near 800, to account for closing and opening connections.

A TCP connection is uniquely defined by local and remote host and port. As such, the router could establish up to 65,535 connections to the same host and port. At the same time, it could establish another 65,535 connections to another host and port.

That means you can have an overall amount of (local addresses) * (local ports = 65,535) * (remote addresses) * (remote ports = 65,535). Of course, some local ports may not be available (services hosted or port forwarding). However, it’s still more than you’ll ever need.

Naturally, this would require a network larger than /24, which poses no problem with OpenWrt and the like. Without aftermarket firmware, most routers are limited to a /24 network, leaving 253 IP addresses for hosts. Wireless connectivity might be further limited, sometimes greatly.

However, you’ll most likely run into resource exhaustion before you’ll get anywhere near these limits. Connection tracking is very hard.