Can a Scrum Master serve more than one team?

Yes, it’s possible. In many organisations, a Scrum Master looks after two or three teams concurrently, especially if they are mature and already self-sufficient in applying Scrum.
However, the quality of support diminishes as the number of teams increases. In complex environments, or with teams just starting out, it is preferable for the Scrum Master to be dedicated to a single team to ensure effective coaching, facilitation, and impediment removal.