A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.

Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them   on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).

The number of Elementals you control is counted as Omnath's first ability resolves. If Omnath is still on the battlefield, it will count itself.

You may choose Omnath as the target of its own second ability.

If the target Elemental is an illegal target by the time Omnath's second ability tries to resolve, the ability doesn't resolve. You don't draw a card if you control eight or more lands. If the target is legal but can't have a +1/+1 counter put on it (most likely because an opponent controls Blightbeetle), you do draw a card if you control eight or more lands.