Depends on my use of the gun and the price of the magazines.
My (former) main choice for carry, a Glock 36, I keep 5 mags for. I've used the gun in a few training classes where mags save time and keep everyone happy. I've shot some "action shoots" with it where I needed X number of rounds and had to have more mags available. Generally, though, I worry about having 3. 1 in the gun, and 1 or 2 extra to carry.
My 1911, I have 6 mags for. Got some extra for competetion shoots, and the price is right.
Some of my non-carry, just-fun-at-the-range guns, I only keep 1-3, depending on what the gun came with and I could readily get.
With revolvers, the speedloaders are usually pretty cheap, so I keep 5+ for them. Generally get them at arounf $7 a pop or so, if I remember right. I haven't bought any for a while.
My revolver moon clips, on the other hand, I keep 25-50 of. You can get them in multi-packs, and I always worry they might bend or snap. Have never had any problems with them yet, though.
“There are three reasons to own a gun. To protect yourself and your family, to hunt dangerous and delicious animals, and to keep the King of England out of your face.” - Krusty the Clown