One? - since i would consider them "light walls" - but they aren't solid blue?
I played it using the rule that one damage marker destroys them...
We played that map for the first time yesterday and wondered about that too. We decided to make every wall on the map a "normal" wall though, although a light wall would make perfect sense too for the wagons.
All walls on the underground map are normal walls.
So all the walls are normal, 2 point structure walls.