From Puzo page 222

(circa 1935-1937)

“There were five or six “Families” too powerful to be eliminated”
“It was Brasi operating alone, when one of the six powerful families tried to interfere and become the protector of the independents….”

– which imply at that time, there were six powerful families plus the Corleones. – which would make sense in a historical sense being that in the real world there were five main Sicillian/Italian gangs and Louis “Lepke” Buchalter’s operation. The others being what have become today’s modern five families, with Buchalter’s operation being assumed by the other five.

Puzo- Page 249
“The war of 1947 between the Corleone Family and the Five Families combined against them proved to be expensive to both sides.”
Page 251
“The New York five families had no desire to make more enemies by going into Vegas after Freddie.”
Page 254
“The Five Families and the Corleone Empire were in a stalemate”
Page 271
“The Don must take command or order Hagan to surrender the Corleone power to the Five Families”