Best in the West
No, it really isn't.Expecting anything above 10 characters is crazy.
From the sounds of it, we aren't going to see a new game for quite a while. From hints of them making a Marvel fighting game and Injustice 3, a new Mortal Kombat won't be for another 6-8 years possibly.
Plus, they always increase DLC counts with each game.
MK9 = 4 characters
Injustice = 6 characters
MKX = 8 characters (9 including Goro)
Injustice 2 = 9 characters (10 including Darkseid)
With the first Kombat pack being confirmed to contain 6 characters, it's really not crazy to assume the second Kombat pack will also contain 6 characters as well, which totals at 12 DLC characters.
18 doesn't seem likely though. They'll probably have two Kombat packs like they did in MKX. If that is the case though, then we still don't know what 3 of the DLC characters are.(Or 9 if by some miracle we get 18.)