After reading through the thread, I must say there have been some great answers posted here.
There are a few things that come to mind for me and this is how I feel.
#1 is shot placement, always. How good of a shot are you? How often do you practice? I consider myself a "good shot", but I also know that when SHTF things will be a lot different than standing still in a controlled environment like a shooting range. The BG will not likely stand still 21 feet away while you empty the gun into him.
#2, what kind of gun do you carry? What caliber? How many rounds does it hold? What is the ballistic rating of the ammo you carry? I come from the "go big or go home" camp. About the smallest caliber I carry is a .357 Magnum(or a .44 or a .45). After shot placement the next most important thing for me is bullet that will penetrate and expand.
#3 is your location. Do you live in a high crime area? A large metropolitan area where the police are likely to be reasonable close, or a very rural area, where your response time is likely to be a lot longer? Do you often travel to unfamilar places?
These things are only some of the things to consider, since crime never makes an appointment, and bad things can happen to good people anytime, anywhere.
At home I tend to feel more secure, even when I'm alone and carrying my own firearm and knowing there are at least 6 more loaded handguns along with an AR-15 and two 12 guage pump shotguns within fairly quick access. Outside of the house I have one(sometimes 2) and at least one reload for each, and that is my minimum.
I doubt that if I ever have to defend myself or my family that I'll ever be upset about shooting too well, having too big of a gun, or having to much ammo available. The reverse could be a different story.