Maybe four survivors? It's apparently possible to do a 'perfect failure' run, where your only survivors are Thane and Mordin, and we know what happens to them in ME3.
I think you have to have at least 3 or 4 live at the end of ME2 to proceed to ME3 with that particular save game. You can't kill Liara in ME2 and temporary squadmates like her do not count towards your end game total and whether or not your save game will import into ME3. In ME3, everyone can actually die by the end of it, including Liara. Although, the run on the beam at the end is the only place she can die.