This is precisely why I carry 50 rounds: 17+1 and two spare 16 round magazines. Karl Rehn, multi-Grand Master USPSA, recommends loading double-stack magazines with one less than the maximum as it makes them easier to properly seat while under stress. Training and USPSA competition has shown this to work well for me. One of the main reasons I chose the S&W M&P9 M2.0 5" as my one pistol I shoot well is its 17+1 capacity. Then two spare for the reasons Chris stated, each loaded one down. 50 is not a magic number of rounds, it just happened as a result of other decisions.The Annoyed Man wrote: While I agree that it is possibly unlikely that I would need to expend all 16 rounds in my G19 in order to affect a stop, it is also possible that I might—under the stress of the situation—shoot to slide lock and find myself winchester on ammo. I hope that I’d have more control than that, but I’ve never been in a gunfight, and I won’t know for absolute certain how I’d respond until the day it happens—which I pray never comes. I almost always carry 2 spare mags with any gun I carry. I don’t carry that many because I expect to expend 46 rounds in a fight. I carry them because (1) I may wish to perform an administrative reload after the shooting stops, and (2) I acknowledge the possibility, however remote, of a magazine-related malfunction. I carry 2 spare mags even with low cap guns like my G43, and I carry 2 speedloaders or speedstrips with a revolver.
I am a coin holder in KR Training's Defensive Pistol Skills Program https://www.krtraining.com/defensivepis ... ogram.html, which includes multiple force-on-force classes, vehicle/parking lot fighting, and trips through the shoot house. Additionally, I've trained on force-on-force, managing unknow contacts, and fighting from, around, and into vehicles with John Murphy, Chuck Haggard, and Hock Hochheim. All that to say I agree with those who have stated the importance of realistic training that forces you to think and decide under stress.
Two books I recommend that help us understand the physiological and psychological stresses of a gun fight are On Combat and On Killing by retired LTC Dave Grossman.
https://www.amazon.com/Combat-Psycholog ... 0964920549
https://www.amazon.com/Killing-Psycholo ... 0316040932