A 4x4 foot garden bed, for example, will have 16 planting squares (because there are 16 square feet in a 4x4 bed). A 4x8 foot garden bed will have 32 planting squares.
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.
The first step is to calculate the dimensions on the inside of the garden bed, which is the length and width that is actually usable for planting. If you bought your beds from a commercial source, then chances are, the interior dimensions are probably a little less than the advertised size.
Another approach is to use concrete blocks, although less aesthetically pleasing, they are inexpensive to source and easy to use. On the market are also prefab raised garden bed solutions which are made from long lasting polyethylene that is UV stabilized and food grade so it will not leach undesirable chemicals into the soil or deteriorate in the elements.