There are currently 59 units of the U.S. National Park Service designated “national park.”
This list comprises the first 28 of the 59 national parks (through 1950). The year given is when the site was established as a national park — some were national monuments or another designation before becoming a “national park.” National monuments could be (and were) proclaimed such by the president; it took an act of congress to authorize a national park.
How many have you visited? (In my case, 24.)
Yellowstone — 1872
Sequoia — 1890
Yosemite — 1890
Kings Canyon — 1890 (originally General Grant NP; renamed 1940)
Mount Rainier — 1899
Crater Lake — 1902
Wind Cave — 1903
Mesa Verde — 1906
Glacier — 1910
Rocky Mountain — 1915
Haleakalā — 1916
Hawai’i Volcanoes — 1916 (Hawai’i Volcanoes and Haleakalā originally Hawai’i NP; separated in 1961)
Lassen Volcanic — 1916
Denali — 1917 (originally Mt. McKinley NP; renamed 1980)
Acadia — 1919 (originally Lafayette NP; renamed 1929)
Grand Canyon — 1919
Zion — 1919
Hot Springs — 1921
Bryce Canyon — 1924 (originally Utah NP; renamed 1928)
Grand Teton — 1929
Carlsbad Caverns — 1930
Great Smoky Mountains — 1934
Shenandoah — 1935
Olympic — 1938
Isle Royale — 1940
Mammoth Cave — 1941
Big Bend — 1944
Everglades — 1947
The National Park Service itself wasn’t created until 1916.