There is most definitely more than one per region in my experience, and I am not sure if there is a limit, I have seen 3 in one system too. They continue to spawn new ones even with people not blowing up existing ones. I have taken out two of the four that was around me recently and I had that set to any distance.
Still 3 in our .5 system at the moment and they have killed at least 3 player structures and got others into the second timer… Great work CCP now we have to alarm clock it and skip work for NPC attacks…