Enchant creature
Enchanted creature gets -1/-1 for each Swamp you control.
The ability cares about lands with the land type Swamp, not necessarily lands named Swamp.
This ability counts the number of Swamps controlled by Quag Sickness’s controller, not the enchanted creature’s controller (in case they’re different players).
This bonus is not fixed; it changes as the number of Swamps you control changes.