http://tinyurl.com/6ek54n

The above interview is not exclusively an MK vs. DCU interview but it does have some rather interesting information on the game's character totals:

"We have a cast of 20 characters, 10 from MK and 10 from DC, and we front-ended our production of the characters," Boon said. "We did all of the DC ones first because we wanted to get them through the approval process. You send them design sketches of the characters, you send the final 3D model of the character, the special moves... There's a lot of back and forth to get those approvals, so we front-ended it to get ready in case there were any major hiccups."

Thus Boon & co. have already decided all of the characters in this game. So it looks definitively confirmed that 20-22 characters is all.

There's more info on the game in the article as well. The rest of the article is actually really interesting; it's about why superheroes have such a difficult time making the jump to the video game world. Worth a look!