Remember when Sakurai said making a new game wasn't about just adding new characters before Smash 4? People immediately thought we were only getting a couple of new characters. Then we got the first gematsu leak, and people thought those were the only 6.
Fast forward post release, and we have 17 Newcomers? And still have the potential of more on the way.
Smash 5 does have many solid contenders too.
New Mario Rep: Likely Toad
New DK Rep: Dixie or K Rool
New 7th Gen Pokemon Rep
New FE Lord
Inklings
Chorus Kids
Chibi Robo
Takamaru
Maybe New Kirby Rep, Like Waddle Dee
If W101 gets a sequel by some miracle, then Wonder Red
If F Zero gets a reboot, a new rep is possible (Goroh/ Black Shadow)
Maybe a new KI Rep if it gets a sequel (Medusa or Hades maybe)
Dillon could get a larger game in the future, so maybe him.
Alph promotion?
New 3rd Party Character (Like Rayman or Layton)
And perhaps another new IP created sometime between now and the release.
There really are a lot of possibilities right away for new characters in Smash 5. I wouldn't expect a low number. If the first game had 12, second 14, third 18, and now the 4th has 17+, I'd say theres no reason we can't hope for a number between 12-18.