Real life gun fight on video. Note the number of misses.
https://www.youtube.com/watch?v=PPGXrMVLJVM
We must each decide for ourselves which risks we are willing to manage and which benefits matter most to us.
Ref: Rangemaster Firearms Instructor Development Couse Student Manual, Revised Dec 2020, Tom Givens.
As of December 2020, 68 of Tom's students, that he knows of, have been involved in defensive encounters. 65 wins, 0 loses, 3 forfeits. The 3 forfeits were not carrying their guns and died in the encounter. Of these 65 wins, none reloaded during the fight although one shot to slide-lock.
John Correia (Active Self Protection) has viewed over 30,000 real life incidents of violence. He has not seen a civilian reload that affected the outcome of the fight.
This seems to indicate that we will win or lose the fight with the ammunition in our gun when the fight starts.
Correia further says that he is seeing multiple attackers with increasing frequency.
John Daub,
https://blog.hsoi.com/2013/07/11/minimu ... ve-pistol/, 7/11/2013.
... Paul Ford (former Austin Police SWAT member). Paul pointed out that in a gunfight you will do about 70% of your worst day at the range. Think about that: take your worst day (under the ideal circumstances of the range), and now make it a lot worse, and that’s how you’ll do.
Let's take a simplified, worst case scenario.
We know that handguns suck at stopping people. Let's say you can consistently hit the high, center chest of the photo-realistic target two out of three times at 5 yards in three seconds from concealment, on your best day on the range. Let's say it's 1 out of three on your worst day at the range. Using Paul Ford's 70%, this indicates you may likely miss all of your shots in your gunfight.
Let's say that through competent training and diligent practice, you manage to consistently get 4 high-center chest hits on your worst day. Using Paul Ford's 70%, that comes out to roughly 3 hits and one miss in your hypothetical gunfight. Let's say that those three hits stop your attacker. But wait, dude has three buddies. That's 4 shots to stop dude, and 12 to stop his buddies.
How many attackers, at your current skill level, can you reasonably expect to handle with the ammunition in your gun.
I've been tested and timed by Karl Rehn, John Daub, Lee Weems, John Corriea, and Tom Givens. I know my worst and best. I compete in USPSA matches with my EDC gear. I have a decent measure of how many attackers I can handle with the 17+1 in my M&P. My sons carry a minimum of 15+1.
I can't, and won't attempt to, tell you how much ammunition is enough for you. I will say that through training, testing, and competing you can get an idea of your capabilities which will inform your decision of how much ammunition is enough for you.