Have or carry? I think it prudent to have "several" for a gun. Whether that is 4, 5, 10 or more depends on the gun, what you do with it, your wallet, your fears, and so on. You never know when one will be lost or damaged or replacement mags will become difficult, impossible or overly expensive to obtain so I feel pretty good if I have 4-5 mags for a gun. Since many of my guns use the same mags, I actually have a lot more than that in many cases.
Carry...my carry days began with a six-shot revolver and two speedloaders, with extra rounds in the car along with more meaningful armament for those unusual situations. To tell you the truth, only when I let my imagination get the better of me did I think I needed to carry more than 18 rounds on me. Maybe if I had been working in some remote outpost somewhere where backup would be a loooooong way off I would have felt differently.
Then came the ability to carry 7 or 8 rounds in the gun and another 14 on the belt and for most cops and claw abiding citizens within the U.S. it is hard to imagine that not being enough. (I don't find most double stack guns to fit my hand that well). I know people who think they need to carry 17 in the gun and another 34 on their belt and there are others who will double that. The chances of needing anything close to that are so small I see no point, but if they are happy then I don't care.
Not wearing a uniform anymore, a gun and one mag seems plenty to me. Now if I were working for the Gov't. in the Middle East, Asia, Mexico, South or Central America, doing drug interdiction in the Everglades or elsewhere, etc., then things would be different.