That depends, of course, on the nature of the organization and the work, whether the work is full time or part time, how often circles meet, how involved the work is etc. The general answer is: a person can be a member of as many circles as they can be productive members of. For most people, what they can track is about 2-5 regular circles.
Remember that circle meetings are only a small part of the circle’s work – the focus should be on operations.
For more information, check out our handbook Many Voices One Song chapter 2