I have done overkill. I have 8 heated water buckets for 14 ducks. Some are only about 3" tall and bigger around for the call ducks. Some are very tall and hold a lot of water for the big Rouen and the Mule Duck. Some are medium size for the Muscovy and Khaki Campbell and Mallards. They are all spaced on a plastic pallet and everyone seems to do fine. I have several different enclosures for all of them but they all seem to congregate in my Medium sized Pet Gazebo. I have it covered with tarps, it has a covered top and open door with no cover for ventilation. I think they stay warmer being altogether and huddling. They have done great and it was only 17 degrees here in Ohio this morning. Even my two eggs were in there and not frozen by 1pm when I got home from Church. I also have three of the black rubber bowls in another area for days when it gets warmer. It will be 50 some this week on Thanksgiving Day and on Wednesday before Thanksgiving. We never know what weather we will get here in Ohio but after all, it is a 4 letter word! LOL