Even if a single tooth is lost, it should be restored. Missing teeth should be replaced not only due to aesthetics, but, more importantly, due to health reasons.
Tooth loss and accompanying bone loss cause the face to look in a characteristic manner: hollow jaw, thin lips, elongated nose, as well as a lot of wrinkles around the lips and the nose.
In many cases, if one tooth is missing, severe malocclusion and bone defects may be observed. Also, bone loss in the dental socket, i.e. the place where the tooth joined the bone, results from this phenomenon. That is why it is recommended that all missing teeth, irrespective of its location, be replaced immediately.