Browning is spot on...magazines are "expendable" but necessary and our unfortunate "over-reaction" political environment makes availability uncertain in even the best of times.
For pistols, I think there's a distinction from a service-sized pistol over a smaller CCW pistol. Just about every double-stack pistol, I try to reach at least 8-10 magazines. For single-stack pistols, I try to make it 10-12.
Small CCW pistols and pockets pistols are typically more like 6-8 magazines. I think have only four for my diminutive Beretta 21A, but I have 8 for my M&P Shield and 8 for my Kahr CM9. I even have five magazines for my Kel-Tech P32; it's my most used mouse gun and I wouldn't mind grabbing a couple more mags.
With the "capacity" scare, I'm less worried about magazines under 10 rounds which is why I stock more of the double stack mags.
Magazine rotation is a good thing, and for my range training, it starts the night before when I load magazines...I hate wasting time on the range loading magazines and eating up valuable training time.
Christmas is a great time of year to add a half-dozen spare magazines to your stocking...they really do make great stocking stuffers!
The real trick is to set the goal and just buy a couple new magazines ever couple of months; over a year, you can easily meet a reasonable goal of 8-12 spare magazines for your semi-auto handgun(s).
ROCK6