Wikivoyage has information about national parks, both about a host of individual ones, general information on parks in some countries and general advice for hiking and some related topics.
The meaning of the term "national park" varies between countries, and in many countries areas with other designations fulfill much of the role of national parks. Also such areas are handled in these articles.
The number of national parks in each countries varies a lot, with some countries such as Australia, Thailand and Israel having 685, 147 and 69 national parks respectively, while other countries only have one national park, such as Portugal or Switzerland.
See also: Hiking in the Nordic countries