Seven parks, including the busiest in America and one you reach by boat or seaplane.
Great Smoky Mountains anchors this region and dominates it: 11.5 million visits a year, more than double any other park in the country. The rest of the Southeast is quieter and stranger. Congaree is old-growth floodplain forest. Hot Springs is a national park with a bathhouse row in the middle of a working town.
Florida contributes three, and two of them are mostly water. Biscayne is 95% underwater. Dry Tortugas is 70 miles past Key West and reachable only by ferry or seaplane.
Virgin Islands National Park is in the Caribbean, 1,100 miles from Miami. It is filed here because it has to go somewhere and this is the closest mainland region. That is an honest answer, not an elegant one.