I don't think anyone reading this thread really expects to find a definitive answer that will resolve the issue once and for all. Since the reality is that most of us will go our entire lifetimes and NEVER need a gun to defend ourselves or our family. Carrying a 2-round Derringer probably is enough for most of the cases where a gun is needed (apart from the inherent safety risk of a Derringer).
Carry whatever makes you feel comfortable. Personally I feel sorry for those who are so fearful that they need to carry multiple guns, with several reloads, just to feel safe enough to go out grocery shopping. If the threat is truly that great, moving to another abode seems more reasonable than living every moment armed to the teeth. I think that carrying ANY firearm, even a small 22LR, is PROBABLY going to be enough to do the job for most folks in most situations. But this is about an individual's comfort level, plus to some degree a rationalization for buying multiple guns and spending countless hours at the range. If you accept that the most likely self defense scenario will be firing no shots, or one or two shots at point-blank personal defense range, then unless you just enjoy shooting and owning guns it would not make sense to own several, or dozens of different handguns and using tons of ammo and a significant part of one's free time devoted to shooting.
My guess is that most of us on this and related forums own and shoot guns for a multitude of reasons, some rational, some not so rational but emotional, so we enjoy the endless discussions of how many rounds, what is the best or minimum caliber needed, the comparisons of revolvers to semi-autos, and so on and so forth.