minimum or maximum number of supports for each solar panel.
Stephen, excellent question! The answer is unfortunately not going to be straight forward. It depends on many factors, including location, module weight and dimensions, racking type, mounting location on the structure, structure framing type and spacings, roof pitch... It's a little bit involved.
Let's for example say that your location is Crystal River and you're looking to mount modules on your two story home. This being a residence puts you at a risk category two for the area. The design wind-speed at that category is 135mph. Note that your building department may impose a higher velocity than 135, so it's always a good idea to check with them first.
Knowing that is the first step. The second thing you want to know is what is the roof structure, what is the spacing of your framing members, what is the pitch of your roof, what is the exposure?
Let's say the answers to these are known. That we have a shingle roof, with 24" rafter spacings, and the roof pitch is 6/12 with a normal terrain exposure B.
Having all of this information at hand we can often use the support tools of the racking manufacture to help us to determine the correct rafter spacing that will be needed. IronRidge and Unirac are two that come to mind that offer these tools.
With that information at hand it is a great starting point but it's always a good idea to run those results passed your building department's plan reviewer first. The racking manufacture might say you can get away with 72" spacings, but they may enforce 48" spacings.
I hope this brief explanation helps you visualize the complexity of the question. If you get a little stuck, please get in touch with me here and I am happy to walk you through getting the answer to your site specific project so you know exactly how many supports you'd need.