Yeah, 45-55 is the range IMO. The less cuts, the higher the number due to the easiness of transferring character data from Melee and Brawl.
Also, basing things off "character slots" is stupid. Nobody cares about character slots.
As for my exact number guess, here's my educated information and guess:
SSB64 had 12 characters, all of them were new.
Melee had 26 characters, 14 more than SSB64.
Brawl had 39 characters, 13 more than Melee.
Therefore, I predict...
Smash WiiU and 3DS will have 51 characters, 12 more than Brawl.
Edit: To receive less than 45 characters would alienate casual and competitive fans, truthfully, especially with "obvious" character that were absent in Brawl. Mewtwo, K. Rool, Dixie Kong, Mega Man, Zoroark, Little Mac, etc all are deserving of at least a lot of consideration for Smash WiiU and 3DS.
If the roster were shrunk, I would probably go back and refund my copies of the game, just because that's just a lazy way to balance the game, given the 2 game's huge budget (and the fact Sakurai chose to make 2 smash games at once). I know many smash fans would do the same. Heck, many are already indifferent due to Brawl being... well... Brawl-which is strategically and seriously slower, simpler, and stalling-based in all the wrong ways!
Making those fans more indifferent and eventually hateful towards future smash games would only hurt Nintendo.