"It isn't a matter of ‘If the next game has 50 characters, that'll be enough… There is a certain charm to games that have huge casts of playable characters, but they tend to have issues with game balance and it becomes very difficult to fine-tune each character and have them all feel distinctive."This inevitably or probably means that there will be less characters added (Obviously) but by how many? Alright, there were 35 slots and we also know that Sakurai knows that there were 13 character additions since he himself included the 3 pokemons, Sheik and Samus. So... Let's see what he did during E3 with the 3 new comers. In the brawl hype era, the only characters with their own respective trailers are Snake and Sonic. In the first E3 trailer of Brawl, there were 5 revealed characters : Meta Knight, Pit, Samus, Wario and Snake. Everyone except Snake did not have their own character trailer probably because maybe it's stressful or too much work? I don't know. Most likely Sakurai wanted to make the 3rd parties feel special which is the usual business trick because the same thing was done in several other games that included 3rd party content (Asura's Wrath, Soul Calibur IV and also Smash 4 :P ) Now in Smash 4, it seems like all of the 3 characters got their own trailers. What does that mean? I mean sure, the first brawl E3 trailer had more characters.... by 2. Snake already had a trailer so that's by 1 now. I think this had something to do with the fact that Sora had to develop a lot of characters.
Maybe I'm over-analyzing. I'm basing this on something that's too unstable as evidence. I don't know. In my opinion.... I think there will be 5 at worst and 10 at best. Some people say 10 at least but honestly for me, it's 5 at the least.