A typical career path for a Scrum Master will start with serving one team. After a while that team becomes less time-consuming to work with, as issues are resolved and the team takes on more responsibilities itself.
At that point, a good Scrum Master will seek additional challenges. Often the logical next step is to begin working with multiple teams concurrently or from working with more demanding teams or products.
When developing new Scrum Masters, I prefer to put the person in a position to most likely succeed. That will mean working with a team that has neither any difficult personalities nor unrealistic delivery expectations. But, to go from good to great, the Scrum Master will need to learn to work under more complex conditions.
This leads to the philosophy that success is often rewarded with additional challenges.
Mentor
A Scrum Master who has been successful in a variety of different contexts and teams, might choose to move into a role as a mentor to other Scrum Masters. This will especially be true and feasible as the Scrum Master gains skills and experience.
In many organizations, this role would be called an Agile Coach, with the most common job description being that an agile coach coaches Scrum Masters (and their teams).