Well I feel like maybe 4-6 more unique newcomers left to go. Depends on how many Smash directs they plan on doing before release, as that seems to be the way they're revealing their new information.
People are expecting something for TGS in September, which leaves October and November before release. So if they plan on doing a direct each month I'd say 6 unique newcomers and ~6 echoes as well. If I were to predict I'd go with:
-Bandana Dee
-Isaac
-Skull Kid
-Geno
-Pokemon Rep(Either Ballot winner or Gen 7)
-Rhythm Heaven Rep
-Isabelle
-Shadow
-Ken
-Dixie Kong
-Impa
-Medusa/Black Knight
The bottom two of each list are my "extras" that I believe would make it in if we get 6 each. Which doesn't seem too farfetched imo, Sakurai's words made it seem like we wouldn't be seeing many newcomers at all, but now that I think on it, he likely means unique newcomers and not echoes. Otherwise he wouldn't have gone all out and revealed 5 characters in one direct a few months before launch. Plus if we get 6 more, that'd leave the unique newcomer total at 10, which is a very solid number, and we'd get 10 echoes as well. Leaving the actual total at 20 newcomers which is more than previous installments, but effectively less due to half of them being Echoes/Clones, as they aren't being counted as separate characters at this point now that you can merge them with the character they echo.
I feel like Sakurai knew going into this that every returning fighter and ~10 newcomers would be satisfactory enough for players, but the thing we look forward to the most is new characters and more representation, so he thought "What could I do to satisfy even more?" and that's how they decided on the whole 'Echo Fighter' thing. Instead of bringing Lucina and Dark Pit back and revamping them to make them more unique, just simply keeping their same movesets works and also doing the same copy/paste with slight differences for other newcomers would work just as well and while half of the newcomers would essentially just be clones of other characters, we'd be effectively getting more newcomers than normal.