Leonard Nimoy - 2 (though it's a cheat because it's based in Star Trek having Greg Grunberg, and Grunberg was barely in it, only his voice, and deleted scenes don't count).
If you use TMP -> Stephen Collins (Decker) -> The Big Picture (1989) -> Kevin Bacon, he's still a 2, along with all of the TOS cast.
In fact, the only main character in any series that doesn't have a number of 2 is Connor Trinneer. If you're only using feature films, he can't be linked at all.
Does anyone know of any Trek actor that has a number of 1?