How many mobile national monuments exist in the US?
"The National Park Service currently manages 391 sites in the U.S. and its territories. 74 of these sites are classified as National Monuments."
Technically, this doesn't answer the above question at all. Only the Cable Cars in San Francisco are considered a mobile national monument. But, again, technically, they aren't a monument. Instead, they are recognized as a national historic landmark.
The United States has many national monuments. A few of the national monuments include George Washington Birthplace National Monument is in Westmoreland County, Virginia; Little Bighorn Battlefield National Monument near Crow Agency, Montana; President Lincoln's Cottage in Washington, D.C.; and the Statue of Liberty. Additional U.S. monuments include Lincoln Memorial in Washington, D.C.; Mount Rushmore near Keystone, South Dakota; and Vietnam Veterans Memorial in Washington, D.C.