Depends on what I use it for. If it's a range gun, one is enough. IDPA or IPSC, I have 5. Carry gun has 2 or 3.
Some I have more. I went around looking for Beretta 92 mags yesterday. Turns out I have 10- mostly GI mags. I've picked them up for as little as $5. A lot of 1911 mags, too. I've traded/sold a few over the years and never bothered to trade away mags.