When Invaders Explore, if a land is both adjacent to the Ocean and adjacent to a land with a Town or City, do you add one or two Explorers to that land?

You add only one Explorer to that land.

According to the Explore rules:

"Add an Explorer to every land of the shown type which either: • Contains a Town or City; or • Is adjacent to a Town, City, or Ocean."
"No matter how many sources are in or adjacent to an Explored land, you only add one Explorer."
(Page 10)

So, even if a land is adjacent to both the Ocean and a land with a Town or City (multiple sources), you still add only one Explorer to that land.

Source: Page 10

Source: Core Rulebook (pp. 10, 11, 15, 32, 9)

Rulora