18 U.S. Code § 3321 - Number of grand jurors; summoning additional jurors
Every grand jury impaneled before any district court shall consist of not less than sixteen nor more than twenty-three persons. If less than sixteen of the persons summoned attend, they shall be placed on the grand jury, and the court shall order the marshal to summon, either immediately or for a day fixed, from the body of the district, and not from the bystanders, a sufficient number of persons to complete the grand jury. Whenever a challenge to a grand juror is allowed, and there are not in attendance other jurors sufficient to complete the grand jury, the court shall make a like order to the marshal to summon a sufficient number of persons for that purpose.
The provisions of the first sentence are embodied in rule 6(a) of the Federal Rules of Criminal Procedure, but it has been retained because of its relation to the remainder of the text which is not covered by said rule.