No individual may fit and adjust contact lenses, including plano contact lenses, unless the registration requirement of Section 2550 is complied with, and unless (a) the individual is a duly registered contact lens dispenser as provided in Section 2561 or (b) the individual performs the fitting and adjusting under the direct responsibility and supervision of a duly registered contact lens dispenser who is then present on the registered premises. In no event shall a registered contact lens dispenser supervise more than three contact lens dispenser trainees.