Less than 4 mags, but the victims must have been concentrated and trapped. I carry 27 rounds in 2 mags.
If you read the transcript of this news report about an eyewitness soldier, it appears to have been a waiting room, and Hasan, being a doctor, had gained access, sitting behind the "desk" -- probably a counter, then stood up and opened fire. Shooting gallery. Element of surprise. Unarmed victims.
soldiers were sitting in a staging area for medical and dental checks when the group was startled by gunfire. "A person behind counter stood up, and he said, ‘Allah Akbar!' And just opened up on everybody," the father told us.