First let's consider an expert's advice. The gun fighter with the largest number of kills in North America was Franz Alm, a sheriff in the Yukon territories during the gold rush. In a 1998 interview, his nephew, David Alm of Valencia, CA, says that Franz always said, "Hold the gun with both hands. Aim carefully. Pull the trigger slowly. That's how you kill a man."
A well-accepted doctrine is two in the chest and one in the head. Standard pistol magazines only need to permit an attack against two assailants. More is irrational optimism.
Do not depend on an instant kill, or "knockdown." The only "knockdown" shots are the cerebellum (near the rear base of the brain), and the long bones of the legs, which are difficult targets.
Shooting the chest is often fatal, but depends on bleeding to lower blood-pressure to the point at which the target faints. With a small-calibre bullet wound a bleed to faint can take as long as thirty seconds, even if the heart or a major artery is struck.
The bleed-out is more than enough time for a counter-assault to kill or severely injure the shooter. Also, once fatally shot, the target can rationally commit to accept any amount of damage. Chest shots against trained soldiers are extremely dangerous to the shooter, and should be followed by a head shot at closer range.
It is a fallacy that a pistol can knock-down a target from kinetic energy. Any shot with that much energy would have an equal recoil, which would break the shooter's wrist (at least). A standard exercise to train police officers in this fact is to let the officers feel a real bullet impact through a riot shield.