Denethor, Ruling Steward's first ability will trigger only once during your end step, no matter how many creatures died under your control this turn. However, if no creatures have died under your control this turn as your end step begins, the ability won't trigger at all. It's not possible to cause a creature to die under your control during the end step in time to have the ability trigger.

Denethor, Ruling Steward doesn't need to have been on the battlefield when the creature died. For example, if a creature dies during combat on your turn and you cast Denethor, Ruling Steward during your second main phase, its first ability will trigger at the beginning of your end step.