This is a question that should be determined during the treatment plan. A good rule-of-thumb is to place one implant for each tooth replaced.
Other decisive factors for the number of implants needed for success is the quality and quantity of the patient's bone. Equally as important are the existing anatomy of the bone and the financial resources of the patient. (At Kneib Dentistry we try not to let money get in the way of good dentistry.)
Placing enough implants to restore teeth is vitally important to the long-term success of the restoration. Simply stated, the most costly mistake is to have an implant fail because not enough implants are placed to support the teeth. If the number of implants is limited due to financial constraints of the patient, then the implant treatment should be avoided or the type of restoration must be altered.