As pl and others noted, this is an oft-discussed topic. I'm not sure there's ever going to be a definitive answer.
It is a fact that Puzo refers to "five families" and "The Five Families" in many places. And, as in the quote pl reproduced, he refers to "these five men" as if there were five of them in addition to Vito Corleone, strongly implying that there were six families.
It is also a fact that he never names more than five families. [The five that are named are always Corleone, Barzini, Tattaglia, Stracci and Cuneo.
My gut tells me that, since he never named a sixth family, there were five total--including Corleone--and that the references to "The Five Families" either represented Puzo referring to them generically, or sloppy writing, or both.