Two of them, actually. Just as a test, I upped the max default squad to 10, and the mission wouldn't load properly afterwards. Not sure if that's supposed to happen.
Anyway I'm not used to playing that way so that's not an issue for me. What *is* an issue for me, though, is the fact that the fourth member of my default squad is unselectable... by which I mean that everyone else, the other three, have the menu box beneath them -with item slots, edit/remove... but the fourth one, on the right, does not have that box. I recall there being two boxes also, one to the left, and one to the right, that say "Upgrade squad through the G.T.S." or something, but there is only one on the left. Bug? Screenshot available on request.