For those who haven't yet accepted the unfortunate reality of our current political situation as it relates to firearms, here it is in a nutshell:
President Obama broke his campaign pledge to respect the second amendment of our constitution as soon as he was elected, before he even took office. He has publicly announced his intention to ban "assault weapons". I put that phrase in quotation marks because true assault weapons aren't what he really means. What he really means are "semi-automatic firearms that may resemble assault weapons and/or hold more than ten rounds". He's also very eager to put onerous restrictions on handguns, whether or not they are limited to ten rounds. This despite the fact that criminals will be unlikely to obey any new laws because, like, well, criminals don't obey laws.
The ultimate irony is that violent crime isn't a personal problem for president Obama, or any of the major players in the anti-gun pantheon. They all have 24/7 armed protection. Having encouraged violent crime with their weakness and folly, they need it. They obviously believe they have a right to self defense. They just don't believe we do.
Perhaps their lives are more valuable than ours. But I digress. The question at hand is "How many magazines should you have for your defensive firearms, and how much ammunition?"
Magazines are the component of a firearm most likely to malfunction. I've broken the feedlips on one and the baseplates on a couple. I dented one, with no idea how. I'm proud to say I've even worn some out. Magazines are also the most likely part of a firearm to become lost. One of mine is somewhere on the Alaskan tundra, and another is at the bottom of Lake Winnipesaukee. I left a couple at a range, and they were gone when I went back for them. I left one in a friend's car, and the unconscionable swine still hasn't returned it. (Greg, if you're reading this, I'm talking about you.) I've managed to lose a couple more with no idea where.
Without a magazine, your firearm is a single-shot. If your pistol has a magazine safety (that is, if it's designed to not fire without a magazine in it), then it's an expensive rock.
I think its a good idea to have a set of "duty" magazines that are tested for function but not routinely used; a set of "range" mags for practice; and at least a couple spares. For example, I usually carry two spare mags with my defensive handguns, for a total of three. So, a minimum of eight per pistol makes me feel warm and fuzzy. Three duty, three range, and a couple spares.
When I was a young Marine (lean, hard and good-looking as well as young, by the way), Uncle Sam issued me seven magazines with my rifle. I don't know how he settled on that number, but a lot of taxpayer dollars were undoubtedly involved in the study. I'm willing to take his word for how many are suitable for that sort of rifle. We weren't issued spares, but could get replacements through the supply system if we lost or damaged any. And we did. So for each AR15 or M14 clone or the like, think about keeping seven "duty" mags, seven "range" mags, and a few spares. Twenty if you like round numbers.
Some bolt-action rifles are magazine-fed, but they're an exception to my rule-of-thumb. They aren't really intended for fast reloads and don't often hit the dirt and concrete of ranges during practice. Even battle rifles like the famous Enfield were only issued with one, and it wasn't removed except for cleaning. Besides, they aren't the object of any attempted bans yet. If you have a SMLE, one is sufficient. If you have something like a Steyr tactical, maybe three.
Don't be tempted to buy cheap aftermarket mags. They're worse than useless. When in doubt, stick with the those made by the manufacturer of the firearm or issued by Uncle Sam. Buy quality and only cry once.
On to ammunition: Once again I'll differentiate between "duty" and "practice".
For practice, I'm going to go waaay out on a limb here and say that 300 rounds per year for each type of firearm is a reasonable minimum for maintaining competence. That's 300 for your pistols, 300 for your bolt-action rifles, and so on for revolvers, auto-loading rifles, pump-action shotguns, over/unders, etc. Really serious shooters will laugh at that - they burn through more in one Saturday at the skeet range. But then really serious shooters don't need any advice from me. Of course more is better.