Hi everyone. I'm glad this was so helpful to so many. It has been a life saver for us with 13 chicken waterers to monitor daily.
The chickens don't bother the cords at all. I've been using these for years and never had a problem.
As for how many to plug in, that would depend on the wattage your outlet can handle. We plug 4 inside one coop and it works fine, along with lights for dreary days.
We tried an aquarium heater once, but it doesn't work on these types of waterers. It has to be submerged in the water which means making a hole in the fount to put it inside and causes the fount to run nonstop. If you use bucket type waterers that may work.