The number of circles corresponds to the number of sets being considered. Wikipedia article has examples with two sets as well as more than three sets. Further, according to the article, all circles must have a nonempty intersection by definition of Venn diagrams; this precludes figure-eight shape. Euler diagrams, however, don't have to contain "all 2^n hypothetically possible zones corresponding to some combination of being included or excluded in each of the component sets."