Bill,
I find six billets sufficient. That is what I have under 12 of our docks. But we do have one dock which contains enough billets to fill the space under the dock completely. That is the first dock I built on this plan and it was intended as the loading dock when we operated a commercial air service with float planes. I still use it as the loading dock for the boats that carry our customers' baggage to our cabins. But very many people at once, and huge piles of gear are often on that dock, a condition to which the rest of our docks are not subjected.