In this game, zero.
I'm pretty sure their idea was originally to only have a few companions to be SGR compatible, and not necessarily even one for each class.
Bioware has made it clear from the beginning in a lot of aspects of the game that they don't agree with that idea (i.e. not allowing any playable races that aren't near-human). I'm rather happy with the more controlled story approach, though I know that many do not care for it and I understand why. I think they could probably draw up some compromise, though.
Well, yeah zero in game for now
I just see a few problems arising if they don't make every class have a SGR...complaints like "You're forcing me to play this class!"
And I wonder if they'll have complaints if every NPC one can flirt with doesn't have a flirt option for everyone. Really, I wouldn't see a problem with it, they could just say everyone can start it off with a flirt, and a lot of people may get shot down.
Makes me curious how they plan to implement it in the end. As I'm sure others are curious about it.