So theres 26 slots on that select screen.
Lets see... heres a list of playable characters that debuted in the first 3 games:
Mortal Kombat (7)
Johnny Cage, Kano, Liu Kang, Raiden, Scorpion, Sonya, Sub-Zero (Noob Saibot)
Mortal Kombat II (8)
Baraka, Jax, Kitana, Kung Lao, Mileena, Reptile, Shang Tsung, Sub-Zero,
Mortal Kombat 3 (8)
Cyrax, Kabal,Nightwolf,Sektor,Sheeva,Sindel,Smoke,Stryker,
Ultimate Mortal Kombat 3 (2)
Ermac,Jade,
7 + 8 + 8 + 2 = 25
Since this is the retelling of MK1-3, thats pretty much your roster right there. There may be some unliked characters cut out (Sheeva, Stryker mainly) and theres a few other guys they could throw in there as playable (Goro, Kintaro, Motaro, Shao Kahn, or some newer characters).