Death-Priest of Myrkul's last ability will trigger only once during your end step, no matter how many creatures died this turn. However, if no creatures have died so far this turn as the end step begins, the ability won't trigger at all. It's not possible to cause a creature to die during the end step in time to have the ability trigger.

A creature you control that's more than one of the creature types listed in the first ability will get only +1/+1.

Death-Priest of Myrkul 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 Death-Priest of Myrkul during your second main phase, its last ability will trigger at the beginning of your end step.