The National Park Service system contains 63 officially designated U.S. national parks.
These 63 parks are a specific category within the much larger National Park System. The broader system includes hundreds of other units, such as national monuments, national historic sites, national seashores, national battlefields, and national recreation areas.
Yellowstone became the first national park in 1872. Congress later assigned the National Park Service responsibility for many different kinds of protected places, but those sites do not automatically become national parks in the formal classification.
The number 63 therefore should not be confused with the total number of NPS units, which is much higher. A site can be managed by the NPS while carrying a designation other than “national park.”