That countdown system you have should be fired whenever you “kill” another actor? The reason it is not working correctly is because that code is on the NPC. It should be counting down in the Level blueprint or the Player blueprint. Each NPC will have that Object Count variable and it will be set to 10. Whatever is in the blueprint of one is in the blueprint of all copies and its children.
You’ll want to have ONE of those blueprints (Player or Level or Game Mode) keeping Count of how many Objects there are left in the level. Move the code there.