At least 30. The "mandatory" bottom of the barrel prerequisites are already like, 20 by themselves.
Street Fighter's got a mandatory 3 character representatives.. Ryu, Chun Li, Akuma
Darkstalkers is a guaranteed Morrigan.
The core Avengers are Cap, Iron Man, Thor, Hulk. That's another four.
The required X-men reps are Wolverine, Storm, and Magneto. Another three.
Plus we gotta have Spider-man.
That's already a roster of 12 and that's just the boring safe choices that people are gonna call you out on if you emit.
But now we look at what the roster is missing. Only two girls, so let's add one from each franchise. So let's say Felicia/Hsien-ko, Cammy/Juri/Viper, Carol, and Jean. That'a another four so the roster is up to 16. We're also light on villains, so Doom and Sentinel, Bison/Juri/FANG, maybe Blackheart or Modok again, and we're up to 20.
And we still haven't gotten into the Clover games, or Megaman, or Resident Evil, or Devil May Cry for the Capcom side yet, let alone the more obscure series that almost never get mentioned.
An MvC opening roster that's smaller than 30 characters at launch is a fucking mistake.