Well, we can attempt to use Wikipedia to figure this out:
Microsoft Studios - Wikipedia, the free encyclopedia
1. As was mentioned, Black Tusk is one team, which Wikipedia credits for an "Untitled Stealth game."
2. You either forgot or missed Team Dakota, who is responsible for
Project Spark, so we have 2 teams now.
3. Wikipedia lists a third studio as "Project Next Studios," but simply says that they are working on a "Next generation core title." A 2012 job listing I found from a forum states that they were looking for a "human resource professional," who was to be involved with "two of your key internal studios, 343 Industries (responsible for the
Halo gaming franchise) and Platform Next Studios (pioneering experiences on Kinect)." So we can surmise that they are responsible for some big-time game with deep Kinect integration, I guess.
4. We've got another from Wikipedia's list, Kids and Lifestyle Entertainment. They are tasked with a "Next generation fitness project - Original IP." So it'll be more than a
Kinect Sports sequel (which is listed under the Rare team).
5. The last one I notice is Playful Learning, which is given another vague description, this one being "Next generation family project."
So there are 5 for you, though we don't really know what we'll get from them. Wikipedia also links to the 2012 creation of Connected Experiences, who is said to be responsible for "Untitled Project – Design Large scale multiplayer title and help other first party studios," though since I know not of anything they've been a part of, they might be TECHNICALLY considered a "new" studio, from a release standpoint. I can't say if Microsoft Studios Osaka is a new branch or what, but they have no credited titles, no pre-2013 notes on the Wikipedia page, and they are generically cited as responsible for "Unannounced projects," though a brief Bing search shows that they are focused on providing content for Japan.
So there isn't concrete evidence on this all, but is seems like there could be as many as 8 studios releasing first-time content on this console.