When I was working in my plainclothes assignment, we were required to carry at least one spare magazine. Didn't matter if you carried an issued double or single stack duty weapon. (I carried an issued single stack .45 toward the end of my career, which used 7rd mags.) There were times when I was working I was working in some of the more "active" areas, or on a warrant detail, when I'd carry a couple spares.
Nowadays, in my retirement, I typically carry one spare mag, except when I pocket-holster one of my LCP's. I don't often carry a spare LCP mag. Why? A couple reasons. First, the really small LCP is not easy to manipulate at speed to reload, including inserting & seating the mag in the really short grip. The slide not locking back on an empty mag doesn't help. Second, the LCP is what I carry when I don't anticipate the need to be armed.
Now, for road trips when I'm going to be away from home for some time, I often carry additional spare mags (and spare boxed ammunition). Might be 1-3 "extra" spares ... or, some years ago, while on a motorcycle trip through some remote places, I carried something like a dozen spare mags for the only full-size double stack pistol I own.
Depends.