Within 30 days after incorporation the board of supervisors shall appoint a board of trustees, consisting of not less than five nor more than nine members to act as the governing body of the district. At any time after the appointment of the initial board of trustees the board of supervisors may, at the request of the existing board of trustees of the district, increase or decrease the number of members of the board of trustees, but such board shall under no circumstances consist of less than five nor more than nine members.