The announcement is light on details, simply stating that "the new content will include new playable characters, skins and a new environment, all of which will be available in the first half of 2016." Who these characters might be, or how many of them there are, remains to be seen.
With that said, Boon's tease does imply that there will be at least four new fighters, with an image showing four silhouetted figures. Furthermore, NetherRealm recently referred to the four-character Kombat Pack as "Kombat Pack 1", suggesting that there would be another.
Four DLC characters have been released for Mortal Kombat X so far: Jason Voorhees, Predator, Tanya, and Tremor. They're available individually, or bundled together along with some different skins in the Kombat Pack.
Mortal Kombat X came out in March this year for Xbox One, PlayStation 4, and PC. It was originally announced for PlayStation 3 and Xbox 360 as well, but those versions have been cancelled, following delays.
Correction: this story originally said that the cancellation of previous-gen versions of Mortal Kombat X was rumoured, but as Occonomiyaki point out in the comments, this was recently confirmed by Warner Bros.