There are four studios that are definitely affected, while a fifth is in discussions in the form of Arkane (currently working on Marvel's Blade). They're part of the Bethesda side of Xbox though, and it remains to be seen whether they'll end up remaining with Xbox or spinning off as well.
Instead, let's start by rounding up those four Xbox Game Studios teams that are leaving Microsoft:
Now, again, if we just count the teams under the Xbox Game Studios banner (and not those from Bethesda, Activision Blizzard and King), we're left with nine dedicated development studios that XGS continues to retain.
These nine studios are as follows:
In addition to these, they've also got Bethesda Game Studios, id Software, MachineGames, ZeniMax Online Studios, Elsewhere Entertainment, Infinity Ward, Raven Software, Sledgehammer Games, Treyarch, Blizzard Entertainment and King all under their belts - all acquired as part of the Bethesda and ActiBlizz takeovers.
These studios are obviously responsible for the likes of Fallout, Call of Duty, Candy Crush, World of Warcraft, DOOM and so many other massive franchises that have become owned by Xbox over the past few years.