There are really only 4 Sorcerers, since the user of the First died long ago or something. Even so, it's unclear whether there's a Sorcerer that uses the Third so his/her position as a Sorcerer is probably a bit iffy too.
The only known Sorcerers, IIRC, is Zelretch, who uses the Second, and Aoko, who uses the Fifth. Kiritsugu introduces himself as a Sorcerer, but really he's not. He's just a Magus Killer. His wife, Irisviel, was a homunculi based upon Lizleihi Justica von Einzbern, the one Zouken was so obsessed with. However, I don't think she's a Sorcer(er/ess) either, since she doesn't really actively use the Third. Ilya would also be considered a contender for the position of Sorcerer, even though she can't properly use the Third either. Touko isn't a Sorcerer, she's just a Magus that received a sealing designation.
Edit: The classification of True Magic is a magic that allows the impossible to be done, to put it bluntly. A user of True Magic is is a magician, which is referred to as Sorcerer now (possibly to avoid confusion with Magus? Dunno
V). So while it may not necessarily be something unexplainable by science, it simply needs to be something that's impossible to do.