According to USA Today, about 64 million people visited the parks in 2013. 

Most visited parks in millions:  Great Smoky Mountains in North Carolina and Tenn.- 9.4 million.  Grand Canyon in Arizona-4.6 million.  Yosemite in California-3.7 million.  Yellowstone in Wyo., Mont.,Idaho-3.2 million.  Olympic in Washington-3.1

So many beautiful national parks in the United States.  Someday, I would like to visit all of them.  Especially the Grand Canyon, which apparently is spectacular to see.