The military and Law Enforcement training both taught me to double tap, two rounds to the center of mass of each target. So in need more than one high capacity mag.
Actually for a group of animals like this one shot would do the trick. The sound of it would send them running. If one fell down they would scatter even faster. It looked as if the first one was holding a pistol but I didn’t see anything about a gun. The elderly vet surely should have his own gun, though, with a large magazine.
No you don’t need more than one. These vermin are cowards. Why else would they bring 9 to attack an old man. Double tap 4 and I promise you won’t see the other 5.