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.
2009-02-01
Aven Trailblazer's second ability is a "characteristic-defining ability." It applies at all times in all zones.
Flying
Domain — Aven Trailblazer's toughness is equal to the number of basic land types among lands you control.
