In terms of stages I'd imagine this game will probably have around 50. Since Brawl is a sequel to Melee, it's obvious that all the series will at least have the same amount of stages in Brawl represented. So lets break it down then.
Mario - 4
Donkey Kong - 2
Zelda - 2
Kirby - 2
Yoshi - 2
Star Fox - 2
Metroid - 2
Mother - 2
F-Zero - 2
Pokemon - 2
Ice Climbers - 1
Game and Watch - 1
Smash Bros - 2
Classic Stages - 3
So that's already 29 stages guaranteed for the game, now lets add all the new stages to the new series we know about so far.
Wario - 1
Kid Icarus - 1
Animal Crossing - 1
Fire Emblem - 1
Metal Gear - 1
Sonic - 1
Nintendo DS - 1
Icon number 19 series we don't know about yet - 1
So when we add that together that's 37 stages we know will be in the game total. But know this is when things start going even higher. Since Mario had 4 stages in Melee, who's to say that the other popular series like Zelda, Pokemon and Kirby wont have at least one more stage then they did in Melee? And then who's to say that Wario, Animal Crossing and Fire Emblem wont be given a second stage in this game. If Mother and F-Zero have two stages then there's no reason why those three series can't as well.
Then the big one is how many classic stages will be in this game. It's pretty obvious that it'll be more then 3. I'd imagine at the very least we're looking at around 12 classic stages which is 9 more then Melee had.
Adding all those different factors together shows that Brawl having around 50 stages is very likely.