Within 30 days after the effective date of the formation of a district, a board of trustees shall be appointed as follows:
(a) In the case of a district that contains territory in a single county, the board of supervisors shall appoint three or five persons to the board of trustees.
(b) In the case of a district that contains territory in more than one county, the board of supervisors of the principal county shall appoint three or five persons from any county in which the district is located to the board of trustees.