I'll come out and say I have no idea how big the roster is precisely, but it's time to quell some worries.
The character select screen isn't final, but 20 is going to be minimum before DLC. Some incomplete characters are being held back for DLC unfortunately. I was told the game will most likely have more characters than SCV though. But just don't expect 30+ characters or whatever.
Maxi is in by the way. Speaking of which, I have some big news about him potentially.
Take this with a HUGE grain of salt because even my source that told me couldn't 100% confirm it, but the wording and what they were told eludes them to believe that he might be gay on SCVI, apparently?
Don't look too far into this yet. But the story mode is going to be covering Astaroth killing Maxi's crew and close friends in some kind of emotional way, and it was said to me that maybe his lover is going to be in the game to some extent and be a man that was killed to give Maxi even more of a motive against his neverending feud with Astaroth now.
For now, just keep that in mind if it is the case.