Not just to carry, but say you’ve bought a new shootin’ iron and maybe it comes with one or two magazines in the box, but how many magazines do you want to go home with that fit ol’ Betsy?
My preference is for four quality magazines although over time I’ve accumulated more—more than I need really, but what the heck—they’re all bought and paid for.
What do you consider to be a adequate number?
I’ll go with that. 4-5 seems to work for me, that gives me enough so that when I get pissed and slam dunk one that’s not working like I’d like it to in the trash can I still have enough for the zombie apocalypse.
For range trips, I found two minimum so I can be in the groove and not lose momentum, preferably three.
For EDC, one in holster, one off body.
My first pistols have a minimum of four—novice excitement 
Now that I know a bit more, no more than three.
THREE - I always want two reloads - if one’s damaged or defective, I still have two mags in a pinch. I want THREE speedloaders for a revolver - one loose, two in a double-pouch for backup.
For me it depends on the caliber and use. For my 9mm I have three 20rnd and 3 California ones for travel to that country. My 1911’s I have a dozen or so, just to reduce pauses for reloading during classes, for revolvers 3 or 4 speed loaders in each caliber.
I like having 4. One for carry and three for USPSA and IDPA.
For JHP 9mm, usually 4. Carry 1 in the pistol and 1 in my jeans jacket or belt. Out in the woods? I up it to 3, 2 on my belt and 1 in.
For JHP .45, 10 total magazines ranging from 8-10 rounds each. I usually carry 3, 2 on the belt and 1 in.
Curiously, i carry 3 JHP magazines for .380 as well. Usually 2 12s and a 10 in. 6 total .380 magazines.
6 each for my 2 Sigs.
WOW, okay, I guess I am weird. I have 12 magazines to 16 magazines for each gun. I went with one is none and two is one, but who knows six might be none. I know when I went out while in the Marine Corps, I carried nine to twelve magazines, and I guess it is just embedded into me. Having those magazines around makes me feel like that is what I should have.
Concealed? 
On a serious note, due to rumors of attacks on Easter Sunday, I asked myself if church were surrounded and I was the only one carrying, how far would 21 rounds (10+1, spare) take me? Thankfully, they remained just rumors.
I always wantes at least 6 extra not counting the two it came with.
I guess it depends on the gun. If it’s a gun I like shooting, and especially if it takes a unique magazine, I like to have at least 5 on hand. Magazines break, get lost, dropped, stepped on, etc, and if the manufacturer stops making that particular one, your gun is just a paperweight.
Now, if it’s a universal mag, like an AR, then not as much of a worry, but, then again, those kinds of guns are usually “fighting” guns, so having lots of loaded up mags on hand is wise, anyway.
Maybe what I’m saying is, like ammo, can you have too many mags?
My EDC came with three 17-round mags and I walked out with three more when I purchased the weapon, totaling six I came home with the day I purchased it. I have since purchased six more 17-round mags, seven 30-round happy sticks and a 30-round drum. All the mags for my EDC also fit my PCC.
My Wife’s EDC also came with three 12-round mags and we purchase three more at the same time. Six more have been purchased since totaling twelve mags for hers.
I have 10 mags for every caliber I own. Except my AR15s which I have 60 30rnd mags all loaded. My AR10 I have 20 mags for. I have 10 10rnd clips and 15 8rnd stripper clips for my M1 Garand.
One in the pistol and three spares. Nothing magical about number 4. The only reason for the number is that most of the training classes require 3 spare magazines and all my belts are setup this way.
That’s my number if ol’ Betsy is going to be a carry. I label them 1-4 and rotate them to establish reliability. At that point number 3 and 4 are loaded with FMJ and designated for the range. Number 1 and 2 are loaded with self defense.
If it’s going to be a safe queen/range toy, then 3 is enough.
2-10, depending on firearm and use. There are cheaters like owning more than one 92 and a CX4, which all use the same mags. PPK/S get just what it came with, as I almost never shoot it. Same with the $100 thug gun.
I have 4 for each gun , I carry 2 with my 9mm that gives me a total of 37 rounds. For my 1911 45 I carry 4 mags two spares on me and one in my truck gives me 33 rounds.
I have at least 4 for each semiauto. Before they go into “active inventory” each is range tested thoroughly and then cleaned. I also have an ongoing regimen thereafter (annual visual inspection, spring tension test, and a wipe down with a silicon cloth). For my carry guns (P365X ans P320) I have 2 - 4 extras at any given point in time. Each mag is numbered and has a maintenence record. I also have backup springs and followers for my carry guns and the others I shoot most often.
No right or wrong answers I guess.
I imagine having a little extra can help in case new laws come about which restrict us. Chances of a magazine damage or malfunction - good to have spare. Range time convenience. Those who are on more of the “prepper” spectrum might double up compared to the average Joe.
For me, 3 total fits my needs, but that’s my system.