I find it highly unlikely that they'll only do one Kombat Pack. We'll get two for sure. Probably 6 characters per combat pack. So 25+12 should give us 37 characters in the end.
I really, really, really hope that they'll continue the trend they started in MKX and Injustice 2, where one DLC character covered four characters. In MKX it was Triborg, who allowed you to play as Smoke / Sektor / Cyrax / Cyber Sub-Zero. And in Injustice 2 it was the Ninja Turtles, who allowed you to play as Leonardo, Donnatelo, Raphael and Michelangelo.
In a perfect world we would get Chameleon or something along those lines, who would allow us to play as Human Smoke / Rain / Ermac and Reptile. I'm sure NRS would be able to work out the mechanics, so that the four ninjas would still feel unique and not all have the exact same strings and stances.