First, this presumes I have a fairly powerful handgun like my Glock 27 .40 with Winchester T series and not some hard to shoot mouse gun.
If I knew I had hit him real well ('calling the shot' as it's called) then maybe two or three times with a bit of pause after that. If, on the other hand, it happens so fast I can't be sure then I'd shoot till he drops.
Trouble is, it's real easy to shoot 5 or 6 shots a second and it might take you 2 to 3 seconds to even realize the guy is falling. And that's how you hear of cops firing 15 or 20 shots!
This is another reason to carry as powerful a sidearm as you can handle well.
"The government has confiscated all of our rights and is selling them back to us in the form of permits."