We generally recommend your shelf be at least 2" thick when using the aluminum bracket - for a standard installation, we advise never going thinner than 1 ¾ in thickness for your shelf at the absolute minimum.
This is because the bracket itself is 1 1/2" thick, so you will need at least 1/8" of additional material on the top and bottom of your shelf to house and conceal the bracket. Just remember to leave yourself that 1/4" of shelf thickness when selecting your material.
In general, it's best to leave at least 1/4" at the top and bottom of your shelf, since its far too easy to blow out 1/8" with the smallest movement. See below for a diagram of the recommended shelf thickness.