I always carry 2 spare mags extra. A few years ago I would have thought that that would be enough. After reading of numerous cases of Flash (Ah-ah!) Gordon mob attacks, I have starting carrying extra mags. If me and my wife are attacked by a gang of feral humans, we won't be going down alone.
The Galco Miami Classic shoulder holster system comes with a double magazine carrier on the off-side. Makes a great way to grab you sidearm and two spare mags all at the same time.
Can everyone post a picture of you're waist with all these magazines attached?