Only one - in the winter. But it can be a big one if your pond isn't especially deep and infertile. It's winterkill or fish that are stressed enough to have disease problems in the spring. You may have to decide which is more important -- the ice skating -- or the fish.
However I don't open up the water completely in my ponds. Just one side. In fact, I have been told to create an open area off to the side and leave the deep central water alone as a santuary of slightly warmer water for warmwater fish. I had problems from stress from too thourough of a mix of the deepest water one winter.
Presently I run two diffusers in the summer an equal distance from each other and the shore but plan on putting in two more lines with diffusers closer to opposite shores to keep enough water open to prevent D.O. depletion but not mixing the deepest water.
Last summer I only used one diffuser in the center of the pond and then connected to a different airline and diffuser off to the side of pond under 6 feet of water once the lake began to ice over. I had no stress problems the following spring although it was a mild winter.
