“An adequate ScrumMaster can handle two or three teams at a time. If you’re content to limit your role to organizing meetings, enforcing timeboxes, and responding to the impediments people explicitly report, you can get by with part time attention to this role. The team will probably still exceed the baseline, pre-Scrum expectation at your organization, and probably nothing catastrophic will happen.
But if you can envision a team that has a great time accomplishing things you didn’t previously consider possible, within a transformed organization – consider being a great ScrumMaster. A great ScrumMaster can handle one team at a time.”
Source: Michael James, “An Example ScrumMaster’s Checklist” on ScrumAlliance.org