I have 14 x 48 fiberboard(1/2) shelves that I would like to use 10 metal (L shape) brackets under(4 support). If I would use 3 instead of 2 would it support maybe 20-25lbs?
The weakest link will be the screws in the top of the bracket into the wall. Hold the bracket against the wall with one hand, and pull down on the front of the bracket with the other hand, and you will see where it wants to pull away from the wall. Make sure that screw goes into a stud, and you're fine. Since studs are vertical, the lower screw will also go into the stud, but mechanically, that bottom screw does not need pull-out strength.
Depending on what you're planning on putting on the shelves, the 10 inchers should be fine, though I would definitely use 3. Make sure you're getting at least two screws from each bracket into a wall stud, because even though it's not a question of the brackets holding the weight, it's the fact that you're using a shelf that's only a half an inch thick, you still want the brackets to be firmly mounted to the wall. If you're putting books on it, then it will probably cause the shelf to bow, whether you use 10 inch brackets or 14's because the brackets aren't the weak link in the system. If you can, screw a 3/4 x 1 1/2 - 2 stretcher across the bottom of the shelf to give it some rigidity. If you're not planning to put books or something equally heavy all the way across the shelf, then your original plan will be fine, but half an inch is a pretty weak shelf, especially one that's 14 wide.
