How many do you need? What does need have to do with this! This is America, the United States of Freedom! We got God, Guns, mom's apple pie, and Baseball!
Buy more mags! Buy more ammo! Get as many as you want and can afford, and be happy!
As for me, I like GI magazines, with PMag or equivalent followers. I'll never use a GI follower again, if I can help it. I like PMags, too, but I have a stack of cheap GI magazines, so I only have one PMag I got for a normal price last summer. Just happened to have a $20 in my pocket when I cruised through a gunshop that had them for $17/ea. I bought it b/c that's what I did then - if I saw a mag I could use at a reasonable price and had the money, I bought it. To my way of thinking, right now, I'd rather buy ammo or reloading components than touch another mag.
I like Warp's thought process - 18 mags (a rota of three sets of mags - 1 duty, 1 training, 1 reserve). It makes sense, and I may adopt it. Dang, that means I have to buy more!
But I have been operating on a different premise - that a guy can get by with 2 or 3 mags, if he has enough ammo and stripper clips. I keep one in the weapon, and a couple more loaded, and have a stack more in reserve. The reserve mags are the ones that go to the range.
I'm not in Afghanistan or Iraq (anymore). I'm not going to throw a .50 cal ammo can full of loaded mags behind my seat or on the floor by my feet on the off chance that the 7-10 mags on my person won't be enough.
Nowadays, if I'm carrying my AR in my vehicle to go on a hike or hunting, odds are I have 2-3 mags, and that's it. When I hike, one will be in the rifle, one in my ruck or pocket, and the other will probably stay in the truck.