Originally posted by bacon:

bigger swamp areas have more crypts. if you have a swamp close to your spawn it might not be that big. a good way to find crypts is to sail around and you can see the crypts from the shore (green lights). many swamps border ocean water.

Bigger swamp area's can easily have one crypt and a smaller one 3/4. Its all very random.

Also the swamp is indeed often found bordering ocean but crypts may appear anywhere in the biome.