So nominally 2 per town/city.

I grew up near a "city" of 50,000 people, and there were at least half a dozen museums there. I know I visited several of them in grade school, so there's likely more I didn't know about, and there's likely more today.

Then there's places like DC, which has museums everywhere.

I recently did a road trip across half the US - pretty much every small town I stopped in had a museum of some sort.

I now live in the 'burbs of a city with a metro population around 5 million. There are half a dozen museums within a 15 minute drive that I personally know of (they're on my list of places to visit), this without going into the city itself.