At the height of the Galactic Empire, the Imperial Order of Battle called for at least two deepdock shipyards to be maintained in each sector, in addition to as many orbital yards as were needed to maintain its sector group.
By the time of 4 ABY, shortly before the Battle of Endor, major shipyards throughout the galaxy included but were not limited to: 30 civilian shipyards, 31 shipyards reserved for the Imperial military, and only 6 shipyards not affiliated with the Imperials at all. Of these, 27 of all 67 shipyards worked on the Imperial-class ships only, 22 worked on cruisers only, and 18 worked on both.
Planets/systems with major shipyards
Behind the scenes
In Star Wars: Rebellion, the shipyard has an advanced variant, which produces vessels twice as rapidly as the conventional shipyard.