If you reveal more than one Devourer of Destiny from your opening hand, you'll put that many triggered abilities on the stack at the beginning of your first upkeep. As a result, you'll look at the top four cards of your library, put up to one back on top, and exile the rest once for each Devour of Destiny you revealed.

A player's "opening hand" is the hand of cards the player has after all players have taken mulligans. If players have any cards in hand that allow actions to be taken with them from a player's opening hand, the starting player takes all such actions first in any order, followed by each other player in turn order. Then the first turn begins.

Devourer of Destiny's last triggered ability will resolve before Devourer of Destiny does. If Devourer of Destiny is countered or otherwise leaves the stack in response to that triggered ability, the triggered ability will still resolve as normal.