“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.
in this blog post, John Piekos explains how the ScrumMaster and Product Owner roles in Scrum are much more demanding than the Project and Product Manager roles of traditional project approaches. With frequent “potentially shippable product increments”, he believes that full-time effort is required from all members in order to be successful.
In this blog post, Dewayne Washington make an interesting between the ScrumMaster role and the referee of a football game.