Whenever I'm asked how many feeders to put on a pond or lake, I respond with the ambiguous (or cya) answer of -- it depends. Are you trying to nourish the growth of the fish, or simply attract them. About the thickest feeder density I've seen on a recreational pond or lake is 1 feeder to 2.5 acres. The intent in that situation was to attract and nourish the fish -- not the sole source of food, but a primary part. I think in your case, you may have to approach it from the other direction. Determine how many feeders you can afford and then figure out how to make that work for you. Larger feeders are more convenient, but you may not be able to purchase as many which would limit your feed coverage. Another thing to consider might be using Scatter Feeders (conventional deer feeders). If you can get them out over the water they do a great job of feeding fish. They are less expensive than directional feeders, feed over a larger area and, being simpler, they are more reliable. The biggest problem is getting them suspended over the water. The costs may exceed the benefits. It will probably be more difficult to fill and service them as well. However, if you have existing piers, large overhanging trees, etc. then it may be worth looking into.
Zach Sweeney