45 is, imo, a realistic roster total and it is pretty easy to think of how it could be filled.
ones we know about
1: mario, 2: samus, 3: link, 4: zelda, 5: kirby, 6: metaknight, 7: king deedee, 8: pit, 9: lucas, 10: ike, 11: snake, 12: yoshi, 13: DK, 14: Bowser, 15: diddy kong, 16: sonic, 17: wario, 18: pokemon trainer, 19: fox, 20: pikachu, 21: peach, 22:ice climbers
(assuming they dont give zss a roster spot, and assuming sheik is part of zelda still)
ones we can probably be sure are coming back
23: luigi, 24: capt. falcon, 25: ganondorf, 26: jigglypuff
ones that 'may' come back
27: mewtwo, 28: marth/roy, 29: young link (if a different link doesnt take his place, ie masks link or ww link), 30: ness (although i doubt it very much), 31: falco, 32: game and watch
new ones who are definitely going to be there
33-35: at least one more 3rd party character--possibly up to 3 more maybe (insert megaman, knuckles, tails, zero, sora, cloud, simon belmont here)
thats about 35 ish, if everything there is accepted.
other franchise characters who might make an appearance
star fox series
36: krystal, 37: wolf o' donell
zelda series
38: midna, 39: skull kid, masks link/ww link
metroid series
40: dark samus, 41: rundas/or a different hunter, 42: ridley
pikmin series
43: olimar
edit: animal crossing series
44: one of the animal crossing guys (dont know their names)
so ive got about 44 ish, considering they accept every character ive mentioned, not counting any japan only games reps that might be put in. all in all, my guess is that the roster count would be about 35-38 rather than 45. (unless ive missed something really big)...