The Mass Shooting Tracker, a crowd-sourced project of the anti-gun folks at the Guns Are Cool subreddit, lists 203 mass shooting events so far in 2015. Add in the shooting at a Louisiana movie theater last night and you get 204. Incidentally, yesterday was the 204th day of the year.
The Mass Shooting Tracker is different from other shooting databases in that it uses a broader definition of mass shooting. "The old FBI definition of Mass Murder (not even the most recent one) is four or more people murdered in one event," the site's creators explain. "It is only logical that a Mass Shooting is four or more people shot in one event."