my mom wants to put up some shelving in the kitchen, she bought 4 3/4" thick 11.25 wide 48" long melamine boards, and had me make some corbels for them, my question is: how many do i need for each board? they are from pallet wood a little thicker than 5/8ths, total length 11.25x11.25x2.5 (cross piece is 3", just strategically tacked wit 18ga 1.25 nails so i can easily put in screws, not sure if additional information is needed to get the right answer so i thought i should add that
Two would support the shelf fine, but melamine might sag, so three per shelf would be preferred. Be sure you can hit solid framing in the wall or use heavy duty hollow wall anchors. I'd use two toggle anchors (30-50lb rating) per support. You're correct in that you should add some wood screws to augment the 18ga brads in your supports.
There is a tool called the sagulator that will calculate how much support a shelf needs based on material, length, width, thickness, load, etc. If you Google sagulator the site should come up and you can fill in your values and it will return sag info.