It's just, if you look at that character select screen, you'd bet on three more characters to fit in nicely there, making 13. But I think villains could end up going on a different page, meaning we may get three more heroes, and a bunch of villains to come. I'd be a bit sad of those three slots were just for three villains and we're done.
There's still a bit of choice left for each side, but really would like to see Agahnim for villains. Skull Kid could be a bet too. They could definitely do just as many villains as heroes based on the numbers on that screen. 13 each side could easily be done.