The clerk of the court for Norfolk county shall, not less than twenty-eight days before the first Mondays of January and July respectively, issue writs of venire facias for fifty veniremen from whom the court shall select twenty-three grand jurors to serve in said court, who shall serve for each sitting thereof for six months and until another grand jury has been impanelled in their stead.
Mass. Gen. Laws ch. 277, § 2F