You can do it like this:
You can do it in any blueprint outside of the block you’re destroying. Use ++ or --, whichever you find more logical. The red event node will be called every time an actor receives a *Destroy *call. There’s no need to count anything inside the actor being destroyed; that’s unless it must know it for some reason, which is unlikely - it’s getting destroyed anyway… 
note1: you do not need to *SET *a variable after ++/–, those nodes set values by reference already.
note2: in your example, if you call *Destroy *and then try to do another thing after, it will not work - the object is being destroyed