It is one thing to give the right answer in a forum, but are we training that way?
You have to train
to shoot until the threat stops. Just about every pistol match I've been to, you put two shots into the "A zone" of each cardboard target and then move to the next target. You'll put only one into a metal reactionary target. A lot of range training is just two to the chest and one to the head and the pistol comes down to the ready, scan and holster (real easy to do with a non moving six foot tall target facing directly towards you at 7 yards and not firing back).
Change it up and shoot different strings of shots and run failure to stop drills. Two to the chest might get the job done but I wouldn't bet on it. You will do exactly what you've trained to do in a confrontation and you don't want your pistol dropping to the ready position or to the holster when deadly fire is still desperately needed to stop the threat.