Bob,
How big is your pond? In my case, I have 2 diffusers in 12' of water on a .75 acre pond using a 1.5CFM pump. My pump is only about 75' from my diffusers. Ideally, I would like a little bigger pump(2-3 cfm) with the same number diffusers although this has worked great for the last 3 years. I would think you would have some cfm loss in that span of pipe, not sure how much.
Depending on all the above, I would probably max on 6 diffusers.
