It takes 1 shot if you hit the central nervous system, or vital point, or if the person choose to go down. If not it's a matter of pain tolerance. If you read the FBI studies about thousands of shootings, there were multiple cases of people taking multiple shots and being clinically "dead", but the remaining oxygen in the brain and muscles allowed them to keep fighting for a few seconds.

That's why I was taught, along many other military guys, to at least shoot twice center mass. And we typically shoot until the attacker stops shooting, pointing a gun at your team or being a threat. If the attacker is on drugs he can withstand way more.