We know there will be more than 24 in the base game. My bet is on 28 or 30.
Here is where the roster stands right now:
Characters traditionally Heroes
Superman
Batman
Wonder Woman
The Flash (Barry Allen)
Green Lantern (Hal Jordan)
Aquaman
Cyborg
Green Arrow
Nightwing
Shazam/Captain Marvel
Hawkgirl
Characters traditionally Villains
Lex Luthor
The Joker
Harley Quinn
Bane
Catwoman
Solomon Grundy
Deathstroke
Sinestro
Doomsday (Probably Playable, Definitely Fightable)
Characters Probably In Via Magazine Leaks (Shazam, Sinestro, and Aquaman all previously part of this same leak)
Raven
Black Adam
Ares
So far that means we know 20 characters for sure, probably 23.
Others highly rumoured include Captain Cold, Martian Manhunter, and Damian Wayne.
Other, less solid, possibilities are Gorilla Grodd, Ocean Master and Dr. Fate.
DLC is also confirmed for this game.
Not all of these characters have appeared in the comic and I doubt all of them will. However, all of the characters in the comic have thus far been confirmed for the game other than Lois Lane (who isn't a superhero) and Damian Wayne (which is partially why he is "highly rumoured").
The characters not-yet-confirmed that I personally still want to see are Static, Dr. Fate, Martian Manhunter, and Booster Gold.