A recent study reveals that the number of museums in America has doubled over the last 20 years:

The agency responsible for providing federal funding to museums issued a count Monday of 35,144 museums nationwide. That’s compared to an estimate of 17,500 museums in the 1990s. […] Historical societies make up the largest share of museums with nearly 17,000 such groups.

Count of US museums revised, doubling old estimate (Boston.com)