To really understand this issue you have to really understand how the National Park Service operates: not like a single government bureau, but like a fast food franchise. Each unit is part of a larger organization that sets basic standards and practices. Beyond that, each franchise manager (park superintendent) interprets the corporate programs to fit local conditions. While ever Social Security office may operate the same, every national park unit does not. Hence, the idea of how many units there are is inherently impossible to determine: there are too many variables. Witness the dozens of titles used for park designations and the haphazard manner in which they are assigned or altered over time. Hence, Congress has never designated what is and is not a park, it has designated the National Park System.