Once you have realized that you can't predict when a criminal attack will occur and have begun to carry your defensive handgun all the time, it is important to address the issue of carrying extra ammunition. It is generally recommended you carry at least one reload with you. The operative phrase in that sentence is "at least."
The peace officer or soldier will have to carry quite a bit of ammunition, because his job is usually to go to the gunfire. The armed citizen, on the other hand, will generally use his defensive handgun only when criminals block his exit. Regardless, it is important to find a way to comfortably and effectively carry some spare ammunition.
For what it's worth, when I carry my 1911, I have two spare magazines in a double mag pouch. Counting what's in the pistol, that's a total of 22 rounds. When packing a double-action revolver, I've got six rounds in a dump pouch and another six rounds in a speed strip, for a total of 18 rounds. A study of actual gunfights will show the vast, vast majority are resolved well within the ammunition limits I have described.
Still, you undoubtedly have extra boxes of ammo at home. Why not find a way to make a better tactical use of them? Why not put that extra ammunition into your spare magazines and strategically stash a few in your car and around your house? You revolver guys should load up your speed loaders and speed strips and do the same. You will probably never need the extra ammunition to save your life, but it is there if you need it.
In defensive handgun classes, after we teach students how to perform speed and tactical reloads, we tell them, from now on, ammunition management is up to them. How much ammo they carry and when they reload is their problem. That's the way it works in the real world, too.