If I understand you correctly then you could use a trigger volume or actor, when you overlap it, change the collision (enable) on the blocking volume, then spawn your enemies. Counter should be easy to set up too, after every enemy death you just add or takeaway one, then do a check if the number is equal to what it should be and if it is, disable collision, if not spawn another enemy (to avoid being stuck). Of course this is oversimplified.