Canon - 1 or 2
In the Disney-approved Star Wars canon, the only one we're absolutely sure of is Count Dooku. We'll have to wait and see if the new movies introduce any more.
- Master Pong Krell, who appears in The Clone Wars animated series, is an interesting near-miss. Krell was a Jedi Master who wanted to become Dooku's apprentice during the Clone Wars, but he was never able to; he was killed by clone troopers while attempting to sabotage a Republic offensive, intending to gain Dooku's favour.
A second that may count, depending on your perspective on Sith-ness, is Quinlan Vos. Vos falls to the Dark Side in the canon novel Dark Disciple, and allies himself with Dooku in a quasi-partner, quasi-apprentice kind of way. It's not entirely clear whether or not to consider him a Sith, but Dooku at least entertains the notion:
[Y]ou could join with me again. We will defeat Sidious together!"
"I won’t be your apprentice!" Vos started to choke Dooku again.
"No, no!" Dooku wheezed. "A team. Equals."
"Sith don't work that way."
"Are you and I ordinary Sith?"
Dark Disciple Chapter 36
Legends - 7 to 14
Wookieepedia has a list of Dark Jedi who also became Sith, so this seems like a good place to start. According to that list, there are seven individuals who are explicitly identified as being Jedi Masters before defecting, but possible as many as 14 fit your criteria:
- Darth Desolous, from The Force Unleashed game
- Skere Kaan, who gets mentioned in a few books and comics
- Darth Krayt, with a ton of appearances in comics
- Ajunta Pall, from the KOTOR games
- Darth Ruin, who comes up in some of the prequel novelizations1
- Count Dooku
- Kalatosh Zavros, from The Old Republic MMO.
There are also loads who are just described as "members of the Jedi Order" (XoXaan, for instance), who may or may not have been Masters.
- Darth Traya, from KOTOR2, was a Jedi Master who was exiled for her teachings, which were deemed to have caused the fall of other students and led to a lot of Jedi being killed (and setting off the events of KOTOR1). She didn't fall until some time later, and even then she wasn't a typical Sith; she rejected the label herself, despite controlling the remains of Revan's Sith empire, and she was more interested in destroying the Force itself, rather than the schemes of galactic domination and/or wiping out the Jedi typical of other Darths. However, she did use the "Darth" moniker herself, so she's kind of ambiguous.
- Barel Ovair and his family, who are mentioned in the prequel web-series for The Old Republic MMO. Ovair is special because his family was sent by the Sith Empire to infiltrate and spy on the Jedi Order, so he was already a Sith when he became a Master. Wookieepedia has entries for Ovair, his father and his grandfather, who were all in the same boat. Only Barel is explicitly said to have been a Master, although it seems likely that his predecessors reached the rank as well.
- Sora Bulq. Sora is special because he was never officially a Sith; Count Dooku wasn't allowed to have an apprentice because of the Rule of Two, but he got around that by creating the "Dark Acolytes", of which Sora was one. Whether or not you count him depends on if you consider these Acolytes to be part of the Sith, even if they're not officially Sith.
- Quinlan Vos, another of Dooku's Dark Acolytes. Quinlan was ostensibly a Jedi infiltrator in Dooku's ranks, but it's left ambiguous whether he was just staying undercover, or if he fell and was later redeemed. He certainly did some very nasty things, but he was also uncharacteristically (for a Sith) helpful to many Jedi he crossed paths with during that period
- Luke Skywalker himself. Luke fell to the Dark Side during the Dark Empire comic book series, which involve Emperor Palpatine being reborn in a clone body2. Luke, who by this point is calling himself a Jedi Master, plans to pretend to defect so that he can get close enough to kill the new Emperor, but he instead falls to the Dark Side for real and becomes Palpatine's apprentice3. Whether you think this counts depends on whether or not you accept Luke's self-styling as a Master. Thanks to bitmask for reminding me of this case.
1 Although Ruin is first mentioned in the Phantom Menace novelization, which is canon, he isn't identified as a former Jedi Master in that work, hence why he lives in the "Legends" category.
2 Because coming up with original villains is so hard you guys
3 This sounds familiar