One should always have extra ammo! Auto mags can break, revolvers have limited capacity, bad guys refuse to die resulting in more shots fired. A perp hopped up on a stimulant; meth, PCP, bath salts etc. are formidable. They feel nothing.....including hot lead. Trust me, I've been in quite a few wrestling matches with them. I've also seen plenty survive what should have killed them.
Under stress you shoot, you don't count. It's impossible to know how many rounds one fired in a dynamic situation. We've seen it with real shootings & Simmunitions training. The instructor asks how many rounds the student fired. They answers are always far less then what they actually shot. Same for real officer involved shootings.
As a citizen, you aren't obligated to 'take action' legally, but ethically a motivated armed citizen would probably take action & not let innocents be slaughtered. The incident in Vegas after two LVMPD officers were murdered resulted in a citizen trying to act. Heroic action it was. Unfortunately, he was out numbered & he died. However, his actions no doubt messed up their OODA loop & slowed them down giving time for LVMPD to respond.
Trolley Square Mall, SLC UT. Active shooter enters mall & wantonly shoots people. Off duty Ogden PD Ofc. Hammond engages perp & contains him. Again throwing off his OODA loop & keeping him occupied until SLCPD arrives & takes him out. Even then, it still took 15 rds of combined 5.56 & 9mm to kill him. Ofc Hammond had NO extra ammo & admitted later he wished he did. He had only the rounds in his gun.
The Trolley Square Mall shooting could have been any Joe/Jane citizen. Hell, it would have been nice if an armed citizen joined the fray. It may have ended sooner.
As a police officer & firearms instructor, it is ALWAYS good to have extra ammo. No, one doesn't have to wear a "TACVEST" full of ammo, but a reload or two is definitely good practice. Speedloaders are a pain to hide, unless you have the right clothing & pouches for them. I carry two with my Mod 60 & a SpeedStrip or two in a pocket. With these enormous tablet phones everyone is toting now days, most people don't wince at an unusual print in a shirt near the waistline. I just wear looser shirts & have no problem hiding a J frame or Glock 23.
With my G23, I have another G22 magazine in a belt pouch. Two more rounds never hurt. Like my dad always says: "it's better to have it & not need it than to need it & not have it". Don't read into stats etc. about citizen shootings. No amount of numerical analysis will cover every incident. You do not want to be the ONE where things go off the rails & you are not prepared.