We all agree that more than one is better than just one. And the Constitution agrees where it says that we have the right to keep and bear ARMS (plural).
But I think it's also a personal question that depends on your personality and sensibilities, the laws of your state of residence, the crime rate and risks associated with where you live and routinely travel, etc. Here's my own single-data-point as an example. I bought a Baretta model 92S (9mm) about a year before it was adopted by the US military. I carried it only occasionally because it's really a difficult weapon to carry comfortably. After a few years of watching the demographics and crime rate change, I decided to get something that I could carry more easily and comfortably - a Glock model 27 (.40 S&W). Regardless of the type of holster, and of course I have several, a subcompact like that can be comfortably carried all day. And then I started climbing in the Rockies with my daughter every summer. I knew that a .40 S&W weapon was insufficient, so that's why I wound up with a Ruger Super Redhawk Alaskan. My experience, then, is that you buy new weapons as your circumstances change.