I'm expecting around five. Sakurai recently said on his twitter that there wouldn't be too many additional characters due to the amount of work it takes.
https://twitter.com/Sora_Sakurai/status/583402090862551040
Which makes sense. Fighting games typically don't tend to have a huge amount of DLC characters (For example, Mortal Kombat 9 had only four, I think), and Smash 4's roster is already packed to the brim with characters (
53+, yes, that includes Mii Fighters, Mewtwo, Lucas, and whatever else we're gonna get).
Outside of Lucas and Mewtwo, I definitely think we're getting Wolf back. Seems extremely obvious, with him being on the same boat as Lucas and the fact that Star Fox actually has a new game coming out.
But I definitely hope the Smash devs eventually revisit the idea of the Chorus Kids, or at least implement some kind of Rhythm Heaven character. RH is a fairly significant Nintendo SPD/Yoshio Sakamoto developed franchise and it's extremely bizarre how much it's been given the cold shoulder in Smash. It's not just the lack of a character, it's the lack of a stage, the lack of an assist trophy, and the lack of
music. Between the Sneaky Spirits in Smash Run and the fact there's data pertaining to RH within Smash 4's files, it's clearly obvious Sakurai was going to do something more significant with the series, and I hope he comes back to the idea for DLC.