I carry what’s in the gun plus two reloads, whatever that may be.
So with the S&W model 65, that’s 18 rounds. With a Browning HiPower, there are 14 in the gun and two spare mags for 40 total.
Due to the front sight getting harder to find, especially at night, I’m looking at going to a Sig P320 with red dot sight. Even with 17+1 in that gun, I’ll carry two spare mags for it.

There are plenty of reasons to carry more ammo besides the ability to shoot more.

If I use the gun, I want to be able to refill it, and quickly. If I use it and survive, I probably just made a lot of enemies and don’t need to be standing around with a partially filled gun waiting for police to answer my 911 call. With a revolver, I may then have only a couple of rounds at most in the gun...after shooting some guy’s brother/some woman’s meal ticket/a gang’s blood brother, any or all of whom can arrive before the law.

With semiautos in particular-
To me, it’s not the number of rounds but the ability to replace the feeding device. If the gun chokes in some way, I want to be able to swap magazines to eliminate the most likely cause.

So why carry two mags instead of just one? Mostly because it makes little difference over carrying one to carry two. There are other reasons, but that’s the main one.

Like I said, there are plenty of reasons for carrying more ammo besides reloading to shoot more. I only gave a couple, but there are plenty.