How do I detect entities/ when all destroyed x happens

I want to make it so when all my enemies are destroyed on a level and door opens up that allows you to go further in the dungeons. I am thinking about making a variable that make it so that if a big invisible object is touching them it adds +1 but a problem with this is I would have to make a detector for every single room...

System:Compare two values: Sprite.Count = 0