"Hitman: Agent 47" is a 2015 action thriller film directed by Aleksander Bach and based on the "Hitman" video game series.
However, it does not have 12 stories; instead, it follows a single narrative centered around the character Agent 47, a genetically engineered assassin.
The movie is about Agent 47's mission to track down and eliminate a corporation that plans to create an army of superhuman assassins like himself.
If you're looking for a film with multiple stories about hitmen or assassins, you might be thinking of "Smokin' Aces" (2006)
Which features various hitmen and criminals converging on a Las Vegas penthouse for a bounty.
It includes several interconnected storylines involving different assassins and their missions.
Alternatively, "Pulp Fiction" (1994) is another film with multiple interconnected stories, although it's not specifically about hitmen.
It follows various characters in the criminal underworld of Los Angeles and includes hitmen among its ensemble cast of characters.
If you can provide more specific details about the movie you're thinking of, I can offer more tailored information.
It seems you're referring to "Smokin' Aces" (2006), which features a storyline involving 12 different hitmen or groups of hitmen hired to kill a mob informant.
The film is directed by Joe Carnahan and stars an ensemble cast including Jeremy Piven, Ryan Reynolds, Ben Affleck, and Alicia Keys, among others.