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.

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.

Tromp the Domains affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won't gain trample or get a power and toughness boost.

The size of the bonus is determined only as Tromp the Domains resolves. Once that happens, the bonus won't change later in the turn even if the number of basic land types among lands you control changes.