You should fire as needed to stop the threat. You should NOT fire a fixed number of shots simply because you were forced to engage. If merely drawing the gun causes the assailant to run then you don't fire. If the first shot ends the threat, shooting two more is not legal. If the second shot stops the attack, following up with a head shot is certainly not warranted or legal.
Has anyone here been in a defensive shooting situation and had to deal with this? Seriously, not being smart-aleck, I'd like to know. (I have not. I had a little bit of security type training in the Navy, and I've thought about it a lot since getting a CWP). I think most here would agree "shoot until the threat is stopped" whether that's one or all of the rounds in your piece.
What I wonder about is how quickly or how slowly one of these scenes would play out. To an outside and objective observer I think a typical self-defensive shooting is over and done with inside a few seconds. To the person defending him/herself I suspect it feels like ages. Given the surge of intense fear, excitement, and fight/flight chemicals the body will be experiencing, in my mind's eye I see things unfolding as I'd shoot, wouldn't be sure of the bad guy's response, in fact I might not be able to tell if or where I hit him, so shoot again, etc etc. Given the intensity of the moment, to an outside observer it might look like "bang bang bang bang" when to me it would be "shoot, observe, still hasn't stopped him so shoot again".
Thus I could see myself shooting once and waiting way too long after to ascertain whether or not I'd gotten him to stop, or at the other end of the spectrum being so in fear for my life that it'd be bang-bang-bang until the gun was empty. Anyone have any thoughts on this? I guess what I'm trying to say is I'd intend to do what most people agree is right, shoot only until the threat stops, but while actually fighting off an attack I wonder how well I'd be able to accurately tell what's going on.