Every time I get asked a question about which firearm or accessory someone needs I follow up with the question “For what?” I have seen plenty of well trained concealed carry owners walk around with a 5 shot j-frame and none of them feel under gunned. The reality is that in some nightmare situation, you’ll never be full prepared, and sometimes we just have to be practical when we are CCW every day. For most practical purposes, I’d recommend keeping a spare magazine in your gun safe in your home, or keep one in your car if you don’t think you can comfortable carry a backup. When most of us are out and about, the ammunition you have on you is all you’re going to be able to fight with, but keeping something near your car and easily accessible in your home is a good idea.
If you are cop or military, you probable don’t need my advice to know how much ammunition to carry. I’ve noticed that most of the Police in my area have 2-3 pistol magazines on them, but considering that they are usually full size glocks, I think 45-60rds of pistol ammo should easily get you back to your shotgun, M4 Carbine, or back to your radio to call in the SWAT Team. Police these days are more likely to walk into nightmare scenarios and carrying that much ammunition is called insurance.