Ed Boon said it'd be the most new characters of any Mortal Kombat game (like you just mentioned yourself), which implies at least 12. That is a ridiculous amount, and one I was really excited by after seeing how cool the first batch of newcomers was.
It's true we don't have the final roster officially announced yet, but a lot of us are currently basing our expectations off the rumored list leaked recently. It consists entirely of more MK9 veterans, Shinnok, and no additional newcomers besides Erron Black. If it's wrong I'll gladly accept that, but so far it's only been increasingly correct.
Also, the PR announcement for DLC mentioned that the Kombat Pack would consist of "classic and guest characters". That makes it appear unlikely that they're going to introduce any characters by those means.