The powers of the county as a body politic and corporate shall be vested in a board of supervisors (“the board”), to consist of not fewer than three nor more than nine members to be elected by the qualified voters of the county at large, or solely by the qualified voters of the respective magisterial or election district of which the member is a qualified voter, plus one additional member elected at large, depending upon the result of the election held upon the questions submitted to the voters pursuant to § 15.2-603. There shall be on the board for each magisterial or election district at least one member, and he shall be a qualified voter of such district, except as hereinabove provided. The supervisors first elected shall hold office until January 1 following the next regular election provided by general law for the election of supervisors. At such election their successors shall be elected for terms of four years each. Any vacancy on the board shall be filled as provided in § 24.2-228.