5 and 8 both work, and don't look bizarre. If they do 8, if it even becomes possible, the remaining three slots will be used for Pichu, Bowser Jr. and any of the characters that are listed as super-easy (it's all relative) to clone.
However, I honestly don't expect them to go beyond 5 more slots, but a 5x10 grid with 49 characters and a random space does sound very visually appealing.
I really don't think 10 new characters (counting Moy) sounds appealing to the PMBR though. 5 almost sounds like a coding nightmare.
Edit:
I also can't shake the feeling that the whole "Able to add 100 characters" thing sounds a little too good to be true. Even if it wasn't, are there even enough sounds slots to add even 20 new characters or even less without intruding on the multiplayer aspects of the game in some obnoxious way?
I'll never touch Boss Battles again, so replacing Porky, Meta/Ridley (Ridley's just going to use his own files...), Tabuu, Petey and Rayquaza doesn't matter to me at all, but like...I really don't think thats enough sounds to cover every aspect of even 10 more characters (not counting Moy).