I have a calculator on my website (
www.shilalasbeads.com/products.asp) where you can enter the dimensions of your vino. It will return the minimum amount of beads you need.
Many people (but not all) double this amount to add an extra buffer. If you open your humidor alot, or put in a lot of very wet or very dry cigars then the extra beads help the RH return the "normal" that much faster, but the extra beads are not required.
I use double the amount in my humidors (these are beads I purchased from Scott before I took over his business).