In the movie Deadpool, on the bridge fight scene, Deadpool says he only has 12 bullets. But those weapons looks like Desert Eagles, and he has two. So that means they only have six round capacity?
From my research for the Can a single bullet from a handgun kill three people in the way Deadpool did? question, he seems to have not fully loaded his weapon.
Deadpool (Ryan Reynolds) can be seen dual-wielding IWI Desert Eagle Mark XIX pistols as his primary firearms, using them to great effect. His Desert Eagles are current-production Mark XIX models with over-barrel Picatinny-style rails made by IWI in Israel. They are recognizable by their safeties, which are retained from the original production Mark XIX; the U.S.-made Magnum Research version, in contrast, now has the "teardrop"-style safety found on the original Mark I.
The Desert Eagles used in the films are chambered in .50 AE, based on shots of the shell casings (with .50 AE clearly visible) and the lack of fluting on the barrels. Strangely, they are apparently not fully loaded, as he states he has "12 bullets" in the highway fight. He shoots 7 rounds from the gun in his left hand and 5 from the one in his right hand, meaning one gun was fully loaded and the other wasn't. Desert Eagle handguns in .50 AE hold 7+1 rounds. Also, at another noteworthy point, every primer of the spent casings are marked with the number of the round spent (from 12 to 1).
The +1 being a round in the chamber. A 7 round clip is the standard package for this gun, and Deadpool was not using an extended clip. The standard clip even has the rounds numbered on the clip (4, 5, 6, 7).
The extended clips stick out from the bottom of the gun.
As we see multiple times, for budget reasons, I mean Wade have ADD, he often forgets his weapons, so he likely forgot to load the gun up all the way.
That, or as mentioned in the comments, he shot some off before the cab ride.
You can clearly see a real clip used in the movie. It has the 4, 5, 6, 7 holes. Deadpool just underloaded the weapon.