Well, the obvious minimum is one, but there's a good deal of variation, especially if the target is moving and/or firing back.
The James-Younger gang tried to rob the Northfield, Minnesota Bank in 1876. Between the battle in the town and wounds taken while being pursued, three robbers died who were shot 1, 2, and 6 times respectively. Five robbers survived, they were shot 1, 2. 3. 5, and 11 times respectively.
Or to pick a more current gunfight, in 1986 in Florida, eight FBI agents fought 2 armed criminals, both of whom were former military and far more skilled and aggressive than the average criminal. Five of the FBI agents were wounded, two died after being hit multiple times.
Both criminals died. One was shot three times, early in the fight. One shot stuck his gun hand, one was a neck hit, the other a head hit that struck the brain. This left the criminal so badly injured he no longer returned fire, though he was still moving under his own power until he was shot in the head three more times at the end of the fight.
The other criminal had already wounded three FBI agents, two severely, when he took his first 'non-survivable' wound to the torso. Shortly thereafter, he took a second severe, possibly 'non-survivable' wound to the torso, after which he wounded two more FBI agents, one of them severely, and killed two. Finally, the criminal was shot in the head and torso, the latter penetrating to the spinal column. This rendered the criminal unconscious and he died at the scene. He also took several lesser wounds during the fight, including three to his gun hand, and was shot a total of nine times.
So the answer is, sometimes one bullet is enough, sometimes eleven hits aren't enough to get the job done.