I was messing around with the Kindle version. This is how many times each last name is mentioned in the novel:

Corleone 770
Sollozzo 231
Tattaglia 90
Barzini 49
Bocchicchio 44
Cuneo 2
Stracci 2

Couple more:

Clemenza 294
Tessio 153

Not surprising about Cuneo and Stracci, because in the novel, they weren't whacked in the Great Massacre--they appeared only at the Commission meeting.

One thing I noticed is that there's no reference to the Cuneo Family or Stracci Family. It seems Puzo always followed the convention of the family taking the name of its leader, but in those two cases you can't be sure.

Frederico 3
Fred 5
Fredo 10
Freddie 98

I searched Geoff's online script: Frederico 0, Fred 0, Fredo 49, Freddy or Freddie 7.

I think that, in the script, in all but one instance "Freddy" or "Freddie" are spoken by non-blood: Kay, Clemenza, Johnny, and Moe. This is the only instance where blood calls him Freddie:

MOE: ...You goddamn guineas really make me laugh -- I do you a favor and take Freddie in when you're having a bad time, and then you try to push me out!

MICHAEL: Wait a minute -- you took Freddie in because the Corleone Family bankrolled your casino....

So the one time he called his brother "Freddie" is the time he wanted to take Moe's exact words and shove them up Moe's backside. It's a great rhetorical device.