Jamieson, Fausset and Brown on Numbers 11:16-17

16-17. the Lord said unto Moses, Gather unto me seventy men of the elders--(Ex 3:16; 5:6; 24:9; 18:21, 24; Le 4:15). An order of seventy was to be created, either by a selection from the existing staff of elders or by the appointment of new ones, empowered to assist him by their collective wisdom and experience in the onerous cares of government. The Jewish writers say that this was the origin of the Sanhedrin, or supreme appellate court of their nation. But there is every reason to believe that it was only a temporary expedient, adopted to meet a trying exigency.