Scion of Draco
Artifact Creature — Dragon
Domain — This spell costs 
less to cast for each basic land type among lands you control.
Flying
Each creature you control has vigilance if it's white, hexproof if it's blue, lifelink if it's black, first strike if it's red, and trample if it's green.
Greg Staples
Modern Horizons 2 (MH2)
• #431Rulings
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.
(wotc • 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.
(wotc • 2021-06-18)
Rulings
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.
(wotc • 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.
(wotc • 2021-06-18)