California Code, Penal Code - PEN § 904

Every superior court, whenever in its opinion the public interest so requires, shall make and file with the jury commissioner an order directing a grand jury to be drawn.The order shall designate the number of grand jurors to be drawn, which may not be less than 29 nor more than 40 in counties having a population exceeding four million and not less than 25 nor more than 30 in other counties.