I’m told the pistol holds 20 rounds in a “standard” magazine. He probably changed mags, but I’m just guessing. But for sure, each bullet could go through 2 or 3 victims.
He probably had a bunch of mags, and was planning on keeping going while the mags lasted.
That might also be why he had two guns. I can see somebody trying to charge him while he was changing mags, but if he had a second gun in his left hand, he might be able to hold it ready to fire with thumb, index, and middle finger, while pulling the next mag out of his belt with the other two fingers of left hand.