Skyfox2000
War it never changes...
Street Fighter 4 3D had what like 50-60 characters didn't have a lot of stages either. If Capcom can add that many characters on the 3ds so can Nintendo. Didn't Sakurai say they was only going to be two music pieces per stage?Without Subspace, the huge amount of cutscenes, a much smaller song selection, and probably other data saving measures (maybe less stages, less trophies, etc; ) I imagine they could fit quite a few characters onto it, and if the 3DS CSS scrolls between pages, it's totally possible to get 50+.
As for Starter vs Unlockable, I imagine a few will be made unlockable we know about (Luigi, Lucario, Tink, Ike, Shiek, and the third parties seem likely choices)