Well, let's look at it by using SCIENCE!
There were 12 characters in SSB64, with 8 starters and 4 unlockables.
Then in Melee, we had all of the original 12, plus 4 new starters and 9 new unlockables. This nearly doubled the roster, with 25 characters, no cuts.
In Brawl, we had 5 characters cut, making 20 veterans, plus 9 new starters and 6 new unlockables. This added 35 characters, only adding 10 new spots in the roster.
Sakurai has stated they're trying to make no cuts, and are trying to get a ton of characters in. We can confirm there are at least 8 starters (Mario, DK, Link, Kirby, Samus, Fox, Pikachu, Olimar) plus 4 other characters (Luigi, Wii Fit Trainer, Villager, Mega Man) means we have 12 confirmed characters. if we go to logic, we can say that there will be 6 new starter characters. If we include Villager and Wii Fit Trainer as starters, it means that there may only be 4 new starting characters. However, I believe that we will get no cuts from Brawl. That makes 37 spots so far. This means, to get an ideal roster of 42-50 characters, we will get 4-12 new characters. And, looking at trends, I can guess on some info.
-Based on the fact that new starter characters seem to follow a trend of being their own series, I'm going to predict that Little Mac is in the game and is a starter.
-We all think Mewtwo will return.
-I'm including a fourth third-party.
-Every game introduces a new rep for Star Fox and Pokemon.
So those rumors supposedly "confirm" that there are at least 42 characters. I'm going to guess that there is a nice round number of 47-50 characters.