If multiple nontoken creatures you control (possibly including Pawn of Ulamog itself) are put into their owners' graveyards at the same time, Pawn of Ulamog's triggered ability will trigger that many times.

If you control more than one Pawn of Ulamog and a nontoken creature you control is put into a graveyard, each of those Pawns' abilities will trigger. You may put up to that many Eldrazi Spawn tokens onto the battlefield.