You are correct. The number of robots you can connect is only limited by the resources on the computer hosting the master. Network bandwidth will most likely be the limiting resource.

To be more precise, it depend on the computer performance, the network bandwidth, but also the number of free TCP port, as each connection (topic) require a new port.

The number of free TCP port is the only "hard" wall for a lot of connection to the ros master, the other (PC perf and Network) can be increased (to infinite, if you have infinite budget).

But I never had problem with multi robot with a good computer and a good bandwidh (3 robot arm + sensors)

Related: #q305027.

And important to note: the "master" (ie: roscore) is not actually involved in data exchange between nodes. It only facilitates initial setup and configuration. As soon as nodes have setup connections (ie: subscriptions), the master is not needed nor active, so it cannot become a bottleneck.