I worked for Southwestern Bell in the 90s pre-SBC (aka just before remote terminals and dslams became common). COs handled mid-tens of thousands of lines in big cities, for smaller more rural areas they generally covered a single town or less often there were a few in a county where the full county was under 50k people.
In towns, we generally tried to keep loop lengths under 30k feet, but in rural areas that simply wasn’t possible. You’d often find remnants of party line systems in those areas and definitely load coils out the wazzoo. It was “fun” unwinding all that crap to install ISDN circuits and later DSL.
I remember the old hats at the time laughing about VDSL saying “leave it to the nerds to dream up some unrealistic shit where the loop length can be at most 2k feet, where does that exist!?” not realizing a few years later RTs and DSLAMs would mean a significant portion of city and suburban customers would be.
I used to work at BellSouth in outside plant engineering in the early 2000s. That's exactly what it was. Of course, by then any expansion was done via remote terminals and COs were becoming very antiquated.
You’ll see numerous COs in a big city, but they are also pretty widely dispersed throughout the suburbs and rural areas.