User blog comment:The Shadow User/Squadmates in Mass Effect 3/@comment-1691295-20101016024551/@comment-730941-20101016045908

Yeah, that's the direction I assumed the writers would be heading down also. New squadmates for the actual combat (I find it rather implausible that Bioware would allow us to form a squad out of ME2 squadmates; the various possible combinations of weapons and powers would be very hard to do), but very close contact with surviving ME/ME2 characters for most missions that aren't dependent on personal recruitment or loyalty missions.

I felt Mass Effect handled this much better than its sequel - squadmates were acquired through the course of the storyline's progression, not in self-contained missions that, although interesting, had almost nothing to do with the main plot. I'd love it if ME3 returned to a sense of urgency and didn't make Shepard trudge around the galaxy assembling a team (again) in order to confront the Reapers.

And to answer your question, the minimum number of squadmates for Shepard's survival is two. Anything lower than that and he dies. If we combine those two with the surviving ME1 squad, we have a minimum number of 3 (Virmire Survivor + those two) and a maximum of 4. This is why I'm almost certain we'll get 'new' characters (although we may have met a few of them before) for the main squad in ME3.