There is no definitive answer to this question, as there are many factors that can influence how far apart wall surf racks should be.
Some important considerations might include the height of the wall, the size and weight of your surfboard and the overall layout of your storage space.
Ultimately, you will need to experiment with different configurations until you find one that works well for you.
One option when determining the appropriate spacing between wall surf racks is to start by measuring out where you want them on your wall.