Risa

It does depend on how you count it.

For Combs, I was only counting Weyoun as one character, even if he's playing multiple copies of him.* And I'm only counting TV and movies, not video games. This also means that Tallman doesn't get to have her Romulan appearance counted twice because the trading card game turned it into a different character.

For Tallman, I'm not counting any work as a double,** but I am counting her unnamed Starfleet officers that each had the misfortune of being played by a stuntwoman, and therefore tended to die. She gets one redshirt role each in TNG, Generations, DS9 and Voyager. She also plays one of the trilithium thieves from die hard in space, one of the aliens that knocked up a warbird's engines, an immortalish prisoner in the gamma quadrant, a Bajoran nurse, and one of the space succubi that tried to beat Harry Kim with a large phallic object and drain him of his genetic material.

So, by that count, Tallman has 9 roles while Combs has 8.

* Obviously this is a matter of preference and interpretation, and the more you think about it the more you start to open Pandora's box. Are clones with the same look and personality all the same character? What about clones that are wildly different? What about parallel universe versions? Is a doppelganger added to the count? Or a time travel duplicate? What about body swaps or possessions, do they count as being a different character? What about a character who is playing another character in an in universe fiction? What about versions that appear in dreams or simulations?

** If we're going to nitpick, I'd argue that stunt doubles are intended to be seen as the character by the audience, so it's not unreasonable to count them that way, even if I'm not.