I'm assuming 40-50 characters, and I got some reasons for it.
- The Smash Bros series has grown to being almost the flagship of the Nintendo Consoles, over the past years.
- Brawl has about three times the development time that Melee had.
- Brawl has a much bigger team than they had on Melee.
- It was delayed, possibly to include even more content.
- Difference between amount of characters in Smash64 and Melee is about double. Makes it likely that it'll double again from Melee to Brawl
- It's obvious that characters are a big part of the game.
- We already got a lot of starters, meaning, had there been only around 35 characters, we'd probably have seen less starters, so people would have more sense of unlocking stuff.
When I'm looking at it from my perspective, I would take a careful approach and bet on a roster between 35 and 45.
But when I look at it, as if I were to create the game myself, I would be sure to put a lot of emphasis on characters. And that's also how the series has been doing, this far... so I'm just gonna assume that the roster will grow in the same manner as before.
I don't think around 50 characters is really crazy to assume. But 60 is pushing it, I doubt that'll happen.