The second ability is a state trigger. Once it triggers, it won't trigger again as long as the ability is on the stack. If the ability is countered and the trigger condition is still true, it will immediately trigger again.

When the second ability resolves, Endrek Sahr will be sacrificed regardless of how many Thrulls you control at that time, even if it's less than seven.

Face-down spells are creature spells, but they have mana value 0.