They are 152 characters in the game - some of them are automatically obtained after completing specific mission from the main (or bonus) campaign, but majority must be unlocked in the other way.
Several heroes and villains can be get in the New York City area (look: Map of the Characters) - usually you must resolve some riddles to obtain them. 11 other characters can be found in Deadpool's Bonus Missions, during exploration in free play mode. Three final heroes appear at the end of the game, when all other quests are completed.
Some characters must be bought to become playable. To buy specific hero, open character reel after completing the main campaign (Y button on Xbox) and choose your favorite, paying studs.