By now, there are 23 characters, which is nearly as much as Melee.
There are a few characters that are already nearly pretty sure to come.
-Captain Falcon. He's been in both Smash Bros games, has an original moveset, and has got an assist trophy.
-JigglyPuff. He's been in both games, has an orginal moveset, and I don't Nintendo would throw him away.
-Luigi. He's been named in an interview, and I seriously doubt Nintendo would not put in Luigi. He's probably get more original moves, though.
-Mewtwo. He has a completely original moveset, and he felt Brawl-ish in Melee. He'd be perfect.
-Gannondorf. Nintendo said they were still busy making Sheik's and Gannondorf's models for Brawl. So I guess he's in.
-Sheik. Zelda NEEDS this transformation, unless she gets buffed alot.
That makes 28. More than Melee. No doubt there will also be few other newcomers, so lets just guess from 3-5 more, making it about 32. There are also few possible returns, but quite doubtfull.
-Ness. He has been in previous Smash games, so there's a possible return. But with Lucas earning his moveset....?
-Falco. He's a clone, and clones have a very minimal chance at returning. But...Ganondorf wil return, probably with new moves. Maybe Falco will too, also with new moves.
-Young Link. He felt like an entirely differen't person to me in Melee. But chances he'll return are VERY small. TP Link got in, and he was a clone too.
-Doctor Mario. Forget about it. Alternate costume at a max. 0.0001% chance at returning.
-Pichu. He's been kinda deconfirmed now Pika got his goggles, but there's still hope. Maybe from a poke ball?
-Marth. Ike got a similiar moveset to Marth's, but he plays nowhere near the same as Marth. So perhaps there's hope for Marth after all?
-Roy. Marth has gotten small hopes for coming back. How big would Roy's be?
If all those guys would return(small chance), there would be 39 characters. But I guess Sakurai will suprise us with way more than 5 more newcomers. I've got my hopes for Brawl up. But remember that 100 characters don't beat a good game engine.