The Corleone family probably had 15-20 capos at that time. They were based on both the biggest and the smallest of the families (Gennovese and Bonanno), and that's probably what you could expect from either. Tessio, Clemeza, and Pentangeli (and later Joey Zasa) were just the three that figured into the various plot lines.
I've read the book at least four or five times, and of course watched the movie dozens of times, and I'm either completely misremembering or just plain misinterpreted things, but I thought it was made pretty clear that Don Vito had exactly two
caporegimes, Sal Tessio and Peter Clemenza. Clemenza worked more closely with the Don, while Tessio's operations were kept more remote. Sure, there were lots of "soldiers" (Paulie was one we got to know a little), and a whole power structure, but I have trouble with the idea that there were another dozen or more captains of equal status to Clemenza and Tessio and we just never saw or heard anything about them.
Time to re-read the book, I guess. It's been a few years, so I'm about due anyway.
Wouldn't Frankie be a more logical "replacement" to Tessio?
I'm answering this one second (though it was posted first) because it only makes sense based on my reply to Bart.
Tessio's departure was during the time Don Vito was transitioning his power and authority to Don Michael. Vito told Michael that whoever sets up the meeting with Barzini is the traitor. There's a brief conversation between Tom and (I can't remember who) where Tom says that it's actually the smart move, and Tessio was always the smarter one. This is in response to the suggestion that it might have been Clemenza. The fact that those are the only two possibilities mentioned seems to again say to me that there's only the two of them.
To answer the question, though, I had never considered the idea of Tessio's replacement. Michael was restructuring the family business, top to bottom I'm sure, as that would be his way, and I guess I assumed that the position was no longer necessary. I always assumed that the Pentangeli character was created when they couldn't get Richard Castellano back to play Clemenza in Part II. To me the idea was that Tessio had turned at the end of the first movie, and with Clemenza turning on Michael, it was the end of Don Vito's original structure, which in a way was something Don Michael wanted anyway, as he had always meant to convert the family business to legitimate business on every level.