Grand juries in New Castle County shall consist of 15 members, and the affirmative vote of 9 members shall be necessary to find a true bill of indictment. Grand juries in Kent County and in Sussex County shall consist of 10 members, and the affirmative vote of 7 members shall be necessary to find a true bill of indictment. Grand jurors shall take an oath to perform faithfully the duties of a grand juror.