The way I see it is why limit yourself? You obviously don't have to go to the extreme of carrying 33rnd stick mags, but just because my Glock 17 holds 18 rounds doesn't mean I have to use them all.
IMHO carry as much as you can reasonably carry and just pray you never have to use a single round.
It's a personal decision and one that needs to be made on need Vs reality. First, we aren't cops that need to keep the BG's heads down. The more shots fired the harder it is to explain.
I don't like auto's. My burb gun is a 1911 and even though I own other auto's, it's the only one I'll carry for serious business. I rarely carry extra mags on me but like Proshooter, have them in the truck.
I do carry a Taser almost religiously. I also carry Bear spray often and those two things are a lot more likely to get used than spare ammo.
My walking around guns are 44 magnums and one 44 Special that I am fond of. If I feel the need I'll carry a speed loader or two.
I also have BFR's in 45LC/410 and 45/70. Big guns stop fights without a lot of extra shooting.
You can carry all the ammo you want but the more it weighs, the more tempted I am to just not carry at all.
I had a friend who left food and other items out of his pack while on ambush, so he could carry extra ammo. One time he actually had 1600 rounds with him. He wasn't going to get killed because he was out.
He didn't either but that's a little extreme for a trip to Walmart..