For all of you who are shooting around the 30-35 range:
Currently, there are 23 characters that are officially confirmed. For the sake of consistency, we're going to treat Zero Suit Samus as her own character, as Sakurai does on his website, and ditto for Sheik, assuming she'll stay the way she was in Melee.
Now, let's start adding to that number. There are three "No ****" characters, and those are Jigglypuff, Captain Falcon, and Luigi. They've been in since SSB64, they have their respective icons (Really only important to Falcon, but still), they're in. I would also include Ness in this category, but with the inclusion of Lucas, his position is kind of up in the air. I still feel here's in, just not in the "No ****" category. Our total is now 26.
Now, thanks to the interview with Eiji Aonuma, Ganondorf and Sheik are pretty much in as well. Who else from Melee is probably returning? It's logical to say none of the clones in their current form, at least, and if the clone can't be fleshed out, they're out. I'm looking at you, Pichu, Dr. Mario, and Young Link. What about Falco and Roy? I'm not seeing Roy coming back, as he was pretty much a promotional character. Falco was much more popular, but it'd be pretty hard to make him different than Fox, considering Fox himself has a made up moveset to begin with. Let's play it safe (for now,) and say neither are in. Total is now 28.
We have a few stragglers from Melee still waiting. Game and Watch is all but in, even without the strong backing thanks to the icon theory. Mewtwo? Probably. He's the only non-"cute" Pokemon, and if reworked, he could be very interesting, I say he's coming back. I'll also toss Ness in here, because, honestly, it's not so farfetched that he and Lucas can be in. And Marth? Despite his appearance to Marth, Ike is quite a bit different than our blue haired friend, and I say which a slight rework, he could be both fresh and new, with him and Ike leading Fire Emblem. Total is now 32.
So on to the new characters from existing series that are all but shoe ins. Krystal from StarFox was confirmed to be in the game thanks to her voice actress leaking the detail, and while it's not known if she'll be playable, I think the chance is very high. Ridley also fits in this category, both rounding off and filling up the starved Metroid series. And you know what, DK needs a villain, and King K. Rool is pretty popular. Let's count him. Now, that's playing it pretty safe, but still adds +3, so we're now at 35.
We also know there has to be at least ONE more new series, and for the sake of probability, I'm saying it's Olimar from Pikmin. Could be anyone, but I feel he has the highest chances. And don't forget Animal Crossing! They have a stage, an item, music, everything. To say they won't have a character is silly.
So, playing it highly safe, we landed upon 37. That's huge. Wolf, Bowser Jr., Wolf Link + Midna, Lucario, Falco reworked, etc. are all VERY viable characters, so we're exceeded 40 at this point. Now add in characters I'm sure will surprise us, such as from some retro game, or maybe a 3rd party that takes us by surprise, and it is VERY safe to say that a number such as 45 is not only realistic, but highly probable.