So Brawl had a total of 35 characters. 37 if you count Shiek/ZZS, especially since they're separate now. (39 if you count the three pokemon of PT although you had to play as all three and Squirtle/Ivysaur are pretty much gone now).
SSB4 has 29 confirmed character so far. The old characters that haven't been confirmed yet are: Wario, Ganondorf, Ice Climbers, ROB, OPKnight, Falco, Wolf, Captain Falcon, Jiggs, Ike, Ness, Lucas, Game and Watch, and Snake, for a total of 14 more potential characters. However I think Kojima said Snake wouldn't be in Smash 4 (although he is kind of a troll).
Hard to say about the others, but I think it's rather unlikely anyone else would go because Smash4's models aren't really that much different than Brawl's. From Melee to Brawl all the models had to be completely redone, but every returning character in Smash4 pretty much looks similar to how they did in Brawl so it's likely they reused the old models and did some retouching, retexturing, and updating to HD. Or at least used them as a base to remake them on. In any case they likely didn't have to start from scratch which would make it a lot easier for them to bring mostly everyone back. Hell they might even bring Ivysaur and Squirtle back as separate chars and just have a ridiculous amount of Pokemon.
So assuming everyone comes back (aside from Snake, Squirtle, and Ivy), that would put the roster at 42. Also right now there are 6 announced newcomers. And each smash bros iteration has added 14-16 new characters so assuming there are around 10 more unannounced newcomers, that would put the total at around 52.