Two improved by you for an industry. Three improved by you for a corp. > 60% of total instances controlled (improved and owned or city state owned) for monopoly. Hit mercantilism and the resources tab will show you a %. Sea resources are a bit weird since there are many, well spread, instances. Monopolies work multiplied by the number of a resource, but only on civs without access to that resource. So a sea resource monopoly will have a crushing multiplier applied to fewer civs. If you secure the multiplier it's probably game done.