Threshold — As long as seven or more cards are in your graveyard, Centaur Chieftain has "When Centaur Chieftain enters the battlefield, creatures you control get +1/+1 and gain trample until end of turn."
6/8/2016 All creatures under your control when Centaur Chieftain’s triggered ability resolves are affected, including Centaur Chieftain itself. Ones that come under your control or become creatures later in the turn are not.
6/8/2016 If you don’t have enough cards in your graveyard at the moment Centaur Chieftain enters the battlefield, its threshold ability won’t trigger. If the number of cards in your graveyard changes and Centaur Chieftain loses the ability once it has triggered, the ability will still resolve.