Some groundwork to keep the numbers 'clean':

One - Let's stick to speaking actors, not stunt actors or the people in monster suits.

Two - Purely holographic or hallucinated characters count as whatever species they appear as.

Three - Changelings, Prophets, and characters in disguise all count as their 'base' species, not the one they're appearing as.

Four - Hybrid characters only count for one species - preferably the one they most match phenotypically, so Spock would only count as a Vulcan, and B'Elanna Torres as Klingon.

Actor W. Morgan Sheppard has played three. I just found out he played a Klingon in the Trek movie Star Trek: The Undiscovered Country. He played a human in the Star Trek: The Next Generation episode The Schizoid Man. And I just watched this a few days ago on Amazon, he was in the Star Trek: Voyager episode Bliss playing a Delta Quadrant alien named Qatai. But we aren't told the name of Qatai's race or his home planet in the episode.

Ed.

Actress Carolyn Seymour has played 3 races in 2 Star Trek series. She played 2 different Romulans in 2 episodes of Star Trek: The Next Generation. Also on The Next Generation she played a female Malcorian in the episode First Contact (not to be confused with the movie of the same name). Finally she played the holographic (and creepy and unlikable) human Mrs. Templeton in 3 episodes of Star Trek: Voyager.

Ed.