How many teams can a Scrum Master run?
A common question I get asked in class is, "How many teams can a Scrum Master run?" I always start to answer this with an old Agile joke:
A good Scrum Master can handle two teams. An excellent one can handle one.
The point of the joke is that once you know and appreciate what a Scrum Master does all day, you will see that it will take all their time to properly serve a single team. When you are less aware of all a Scrum Master should be doing, it's easy to think that two or more teams would make more sense for one person. With that being said, I commonly see organizations that have Scrum Masters working with two teams at once. My recommendation to them is the same as I'm giving you though, that one Scrum Master for one team is still the best way to work.
In order to see why a Scrum Master works best when attached to a single team, it's important to see what a Scrum Master does all day. There are several articles out there that try and describe a day in a life of a Scrum Master (such as this one or this one). They are all useful in understanding what goes into a typical day for a Scrum Master. I would break these things down very simply into the following groups:
Coaching the Product Owner - The Scrum Master is responsible for serving the Product Owner and helping them to get the most value out of the team with every Sprint and every Product Increment. A Scrum Master then coaches a Product Owner in their role on the Scrum Team. How is the Product Owner working with the Stakeholders and are they jointly helping to define the most valuable items in the Product Backlog? How is the Product Owner working with the Dev Team and are they communicating and collaborating to have a shared understanding of not only what needs to be done by WHY it needs to be done? As I say in class, the Product Owner role is not an easy one. Scrum Masters need to help then to assist their Product Owners to be successful in their role.
Coaching the Development Team - The Scrum Master is responsible for serving the Development Team and helping them to product a high quality, "Done" product increment each Sprint. Scrum Masters do not have to be technical but do have to understand dev practices enough to help their dev teams make the most out of them. Understanding how things like automated testing, swarming, and test driven development can improve the quality of your Product Increments are a necessary part of coaching up your Development Teams.
Coaching the Organization - The Scrum Master serves as a change agent to the organization by helping them to understand how things will change in and Agile environment. In fact, this might be the most important job of a Scrum Master. Organizations all too often ignore the foundation that should be set with things like the Agile Manifesto and Scrum Values. It's not simply the technology group or the product group that changes with Scrum. Leaders must change as well. Agile tends to spread to other areas like finance and HR so a Scrum Master needs to help them understand Agile. Without this, organizations focus on the practices alone and wonder why they don't see the benefits. Agile is a culture change, not a set of practices.
So as you can see, it's a tricky question. Scrum Masters are the glue for a Scrum Team and if you are asking your Scrum Masters to spread themselves too thin, your teams could start to come apart at the seams. My recommendation is that the best practice will always be to have one Scrum Master for one team.