I think that 30 character would be reasonable and could easily fit in the game. There are only 3 sizes of characters so for example if there were 10 small characters (baby mario, baby luigi, toad etc.) they would all handle the same so the only difference would be what they looked like, and their speech. They would all have unique weapons, but look how many weapons are in SSBM.
The 16 characters they have announced are fine so long as at least Boo and Toad can be unlocked. If not why are there all the baby characters instead of different characters like Toad. How can you have Mario and Baby Mario in the same game anyways, they are the same character.
Oh well, no matter what is in this game, the game will undoubtedly be fantastic.