There are several places (including those cited by plawrence and others), in both the novel and the film, in which "five families" seem to be mentioned in addition to the Corleones. This gives the impression that there are actually six families. You can make a good case for six families based on those mentions. But only five--Corleone, Barzini, Tattaglia, Stracci and Cuneo--are ever named. In my opinion, "the Five Families" is a kind of generic reference to the five that are named--even when one of the Corleones says "the Five Families," he's referring to the four plus the Corleones. Another possible explanation is sloppy writing, of which the novel has plenty.