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I always carry at least 4. But, it comes down to what you are comfortable with. I always look at the worst case scenario and plan accordingly. The best case, average scenario is not the one that is going to get you killed. The worst case scenario is the one that will get you killed. Especially if you run out of ammunition. For example, most common, everyday dregs will probably not want to get into a gunfight and go into flight mode. Then there are the ones whose physiology gives them superhuman strength. Consider many police encounters where the bad guy(s) was shot multiple times and continued to shoot at the police, even though their bodies had taken multiple, seemingly incapacitating damage.

An example would be the two armored car robbers who engaged in a shootout with FBI agents in Miami in 1985. They took multiple hits to major organs and continued to shoot at FBI agents and killed several of the agents before they finally died. Someone will come along and say that they're not the FBI and have no intention of engaging armored car robbers. But, the point is, you carry a gun for protection without intending to engage anybody in a life threating situation. You may not intend to use your weapon, but, you carry it just in case you do.  You leave your house and have no intention of walking in on an armed robbery at the gas station. The people having dinner at the Luby's restaurant in Texas had no intention to encounter a nutcase who would drive his pickup truck into the restaurant and get out and start shooting restaurant patrons for no apparent reasons. Who expected that to happen when they went there to eat?

There was a Vietnamese restaurant in New Orleans where they were robbed. The robber didn't just rob them she took them into the back and shot all of them to leave no witnesses behind. Turned out a little girl survived and identified the robber as a uniformed NOL police officer. I'm sure none of the restaurant patrons thought they were going to be robbed and then killed that day.

Or the nut that went into the McDonald's restaurant in San Diego and started shooting patrons for no apparent reason. Or the two terrorists in San Bernardino, CA. If I am out and about and something like this happens, I will be wishing I had even more than the 4 magazines that I carry now. FBI Special Agent Mireles who almost died in the shootout in Miami in 1985 said he learned one thing from that experience. Carry as much ammo as you can.

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Wow..... If you get to the point of needing 4 mags you are doing it wrong.  Pistol is only to get you back to your rifle or vehicle to GTFO of that situation.  

Mag in pistol and spare that is all for me.

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Granted this is about a cop at work, but, even if he tried to retreat, I doubt he would have just him go.  If the bad guy here decided to take you on, I think you would have the same problem as this cop.  
https://www.policeone.com/police-heroes/articles/6199620-Why-one-cop-carries-145-rounds-of-ammo-on-the-job/

I usually carry a spare mag, either a 17 round one for the G19 or an 8 round one for the P290RS.  For either one, it is a DeSantis in my front weak side pocket.