I was talking to someone who had been asked to be the Scrum Master (SM) for four scrum teams at the same time. My initial reaction was that the manager who had set this up has no understanding of the SM role.
That then led to the obvious question of why is it ok for an agile coach to work with four or more teams at once but not for a SM? After all, there’s an awful lot of overlap between the skills required of both roles.
The key differentiator is that the SM is internal and the Coach is external.
The SM is a part of the team and contributes to the teams goals; they are embedded and doing the work of the team. The coach, on the other hand, is always trying to disengage from the team. They’re building up capacity, and teaching skills so that the team becomes self-sufficient and doesn’t need them anymore.
The coach is always planning to leave, the SM is planning to stay. That focus is quite different, and the results from having each is quite different.
So when I hear “one SM for four teams”, that either means that someone is using the wrong labels and what they really want are teams without SM’s at all, and a coach that floats between them, or they fundamentally don’t understand what an SM does for the teams. I find it’s more often the latter.