The number of implants is usually dependent on the quality and thickness of the bone. The stronger the bone the less implants. For example the bone in the lower jaw is typically thicker and stronger requiring less implants; while the upper bone is softer and requires more. More implants will distribute the force and less stress across the bone. In all cases you should follow the advice of your surgeon; if he/she recommends 5 implants, then you should comply with his/her recommendations. However, if there is a choice, he or she likely feels comfortable with four, but is giving you the option of five in order to provide the most stable and durable base possible. The majority of the cases I have seen have been on four implants, and they have a very high success rate.