unit training

Hey. Im recently making a wc3 mod where all four races are replaced by dwarves, goblins, nerubians and one more (not sure which to choose yet :P). I've got an idea to make nerubians make units like zerg in sc2 (or should i say similar). Now i got 2 units for one type (for example Nerubian Drone...

I want to make it where when training a unit, the barracks or what ever i am using will lock it to where I can only train ONE unit at a time instead of being able to spam another unit,