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.
Domain — At the beginning of each player's upkeep, that player gains 1 life for each basic land type among lands they control. Then this enchantment deals 3 damage to that player.
