The Corleones actually may have been the 7th family. The Bocchiccios were probably included at that meeting.

In the movie Don Corleone starts out the meeting saying :

"Don Barzini, I want to thank you for helping me organize this -- meeting here today. And also the other heads of the Five Families --"

Don Corleone is thanking Don Barzini, the head of one of the families, and then in general he goes on to thank the other heads of the FIVE families.

He's obviously not thanking himself, and he obviously thanks them in addition to thanking Don Barzini.

So this line, in addition to what some others have posted above, says to me that there were 5 families + the Corleones and The Bocchicios, which makes it seven total families.

The Corleones were at war with 5 of them, and the Bocchicios were never involved in an actual war as they were the family that made their living being paid to give members of their own family as peace hostages during a negotiation between other rival families at odds.

Don Cardi