Well we have 40 assist trophies right now and Sakurai said there were over 50. If we can take him on his word at this (and really I don't think he'd have a reason to fudge the truth when it comes to assist trophies) then that means we'll get somewhere between 11 and 19 more assist trophies.
It's probably somewhere in the middle of those numbers so I think 13-17 is most reasonable to expect.
That's not a lot when you consider there may be some more returning assist trophies and new assist trophies. We're probably getting a couple more 3rd party guest assists like Bomberman and Rathalos too.
Not that I think every old assist trophy will return, but there are definitely a number that are still strangely absent. Isabelle, Shadow, Isaac, Barbara the Bat, Saki Amamiya, Dillon, Infantry & Tanks, Ray MK III, Chain Chomp, Kat & Ana, Lakitu & Spinies, Elec Man, Excitebikers, Nightmare, Sable Prince, Skull Kid, Magnus and Phosphora are all assist trophies I'd expect to return if they haven't been promoted to playable. (though perhaps Elec Man has been replaced by Zero? It could happen for third parties maybe)
I could see Helerin, Jill Dozer, Devil and Mr. Resetti being cut/not returning though.
I know it's not saying much since we have leaks pointing at Isabelle, but this tells me that there are more assist trophies being promoted to playable. (and perhaps some assist trophies are being removed for no apparent reason, which wouldn't that be that unusual)
Edit: Now that I think about it. Do you think it's possible the mechanic of being able to hit and KO assist trophies might result in some old assist trophies not returning? I know we have both assist trophies you can hit and some you can't, but the emphasis does seem to be on KO-able assist trophies.
So with all of that, basically the point is, I feel as well with how Sakurai is giving in to fan demand generally a lot this time around, that for ATs as well for the major players he'd deconfirm earlier or with tact, or leaving it so that fans can still be hopeful somehow even from the AT pool, causing generally less disappointed fans overall. So if Isaac truly was an AT, I feel if they are aware of his popularity, they'd have at least done so by now. The fact he isn't, gives me hope with this theory.
If that insider information on King K. Rool is to believed, they'd be well aware of who's popular. I could actually see the way they just slipped Ashely at the end of the assist trophy reel being a way to not drag it out for her fans. (then again maybe it was just a coincidence they showed her off?)
I feel like it could also be wishful thinking that they would reveal things in that way. They might not be too worried about dragging things out for fans.