Total number of anchors: Add the results of Step 2, and multiply by 2. In my example, (7+3) * 2 = 20 anchors.
Number of anchors per board: (Usable Length / Spacing for anchors) - 1. For my beds, I need 7 separators for each 8 foot board and 3 for each 4 foot board.
They are perfect for the disabled gardener, those suffering from arthritis, and the elderly as they can be elevated to a suitable level so that bending over and kneeling will not be necessary.
Then, determine what you would like to use for the anchors. If you built your garden bed out of lumber, you could use nails as anchors, spacing each nail the appropriate distance as determined by the results of the calculation in Step 1 above. If your garden bed is made out of a composite material (part wood waste, part plastic), I recommend using an anchor placed on the outside of the bed. Since my beds are 6 inches high, I used 12 inch metal spikes I bought from the local hardware store, pounding them into the soil directly on the outside of the garden bed.