Domain — {T}: Prevent the next X damage that would be dealt to target creature this turn, where X is the number of basic land types among lands you control.
2021-06-18
Domain abilities count the number of basic land types among lands you control, not how many lands you control or how many of any type.
2021-06-18
The basic land types are Plains, Island, Swamp, Mountain, and Forest. Land types other than basic land types (such as Desert) don't contribute to domain abilities.