A licensed contracting master plumber, master plumber, or journeyman plumber must be present and directly supervise all work performed by an apprentice plumber. A licensed plumber shall supervise no more than three apprentice plumbers at a job site at one time. More than three apprentice plumbers may work at the same job site at the same time as long as the ratio of licensed plumber(s) never falls below one licensed plumber for every three apprentices.