Mass. Gen. Laws ch. 221, § 53

The governor, with the advice and consent of the council, shall, as vacancies occur, appoint masters in chancery, who may act throughout the commonwealth, so that the number thereof in the several counties shall be twelve in Suffolk, eleven in Essex, eleven in Middlesex, seven in Worcester, six in Norfolk, six in Plymouth, six in Hampden, and not more than five in any other county. They shall be sworn, and shall hold their offices for five years.