To ensure the shelf is at the required height place a small block of wood on the brackets level with the pencil line and mark the fixing holes. Drill and plug the wall then hold the brackets in place whilst the screws are fitted, fit all of the screws before tightening them fully. Once all the brackets are fitted, place the shelf on the top of the brackets, making sure the overhang is equal at both sides.
http://www.diyhowto.co.uk/projects/shelving.htm
There probably isn't a 'rule'... but common sense says the more brackets, the better. I would recommend putting a total of 5 or 6 brackets, securely attached to both the wall and the shelf. Affix to studs if you can, evenly spacing the brackets along the shelf. Be sure your bracket is at least 5 (more than half the depth of the shelf) to keep it from 'flipping'. If you can, run a shallow groove down the length of the shelf, about an inch from the back.
https://answers.yahoo.com/question/index?qid=20071008070038AABud35
* Hold the second pilaster against the wall with its bracket in place, then lay one of the shelves across the two brackets with a level on top, as shown in the drawing. * Move the second pilaster up or down as necessary until the shelf is level, then use a pick to mark the location of the top hole on the wall. * Set the shelf and the second pilaster aside, then drill the hole for the first wall anchor at the top. Mount the pilaster with the top screw only, then proceed to finish installing the second pilaster as you did the first one.
http://www.nytimes.com/1987/02/08/nyregion/home-clinic-how-to-put-up-shelves-and-make-sure-they-re-level.html
What type of wall are the shelves being fitted on, is it brick or is it a studded wall, what is going to be stored on the shelves, take into account the size and weight of the items and also the clearance needed between additional shelves.
http://www.diyhowto.co.uk/projects/shelving.htm
Mark the hole positions, then drill and plug the wall, fit the batten and after ensuring they are both level fasten all the screws fully. Fit the shelf and after making sure it fits properly screw the shelf to the brackets using countersunk screws. Corner shelves are a great way to utilise space that is usually wasted.
http://www.diyhowto.co.uk/projects/shelving.htm
In most homes, measuring from the floor or the ceiling is not enough-all too often floors and ceilings are not level. Here is the best way to insure a level installation: * After the first pilaster is installed, insert a shelf bracket in one of the slots near the top end.
http://www.nytimes.com/1987/02/08/nyregion/home-clinic-how-to-put-up-shelves-and-make-sure-they-re-level.html
apart on level line or use templates #99037 and #9038.End brackets to support the front of the shelves: Install on the same level line as the wall clips and center on the front rods of the shelves. All shelves must be front supported a maximum of every 36 in.
http://www.closetmaid.com/en-us/Documents/Wire%20Shelving%20Specifications%20rev_6_12_English_HQ.pdf
If the shelves are secured with wooden brackets, you usually screw through the top of the shelf and into the bracket to secure the shelf, use countersunk screws so the head will not protrude above the top of the shelf.
http://www.diyhowto.co.uk/projects/shelving.htm
Supporting brackets for most shelves should be spaced no more than 24 inches apart for light to medium loads (15 to 20 pounds per foot). This means that the vertical slotted pilasters must be spaced at about that distance from one another on the wall.
http://www.nytimes.com/1987/02/08/nyregion/home-clinic-how-to-put-up-shelves-and-make-sure-they-re-level.html
Best Answer: There is no set rule it varies with type of shelf, bracket, and weight of items. I would recommend 5 brackets for this instillation one on each end the rest on the studs in the middle (assuming 16 inch centers) just make sure you anchor the brackets into a stud and you will have no problems.
https://answers.yahoo.com/question/index?qid=20071008070038AABud35
what is the spacing for shelf brackets
The spacing should be about 24 inches apart for light to medium loads or 15 to 20 pounds per foot for shelf brackets. Shelf brackets should be spaced 24 inches apart for light to medium loads or 15 to 20 pounds per foot.