Not all teeth need to have brackets bonded during orthodontic treatment. It depends on the number of teeth that have to be moved around. Sometimes, some of your teeth are already in a good position, and they do not need to be shifted. Sometimes, the teeth that are in a good position need to have brackets placed on them because these teeth provide anchorage for the movement of other teeth. If not enough brackets are employed, the teeth may shift uncontrollably because there is insufficient anchorage. Anchorage provides stability to the orthodontic appliance/brace.
See also: Short & long roots