Failure to file apportionment plan; election of board members at large. If no apportionment plan is filed with the county clerk as required by this Division, the members of the county board shall be elected at large in the county. If the county board has determined the number of members for the board, that number shall be elected; otherwise, the number of members to be elected shall be the largest number to which the county is entitled under Sections 2-3002 and 2-3002.5.