Support for butcher block desk
The site you linked states "space the brackets 18 - 24 inches apart" source 1 source 2, with the first 4-6" away from the edge. Because you definitely don't want it to flex, even under abuse, I'd err on the side of caution.

With a 8' desk:


24" apart would be 4 brackets.
18" apart would be 5 brackets.
16" apart would also be 5 brackets.