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.

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

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.