Charles Ellicott Commentary Numbers 11:16

"And Jehovah said unto Moses, Gather unto me seventy men of the elders of Israel, whom thou knowest to be the elders of the people, and officers over them; and bring them unto the tent of meeting, that they may stand there with thee." — Numbers 11:16 (ASV)

Seventy men of the elders of Israel ... — We find mention of elders of the people in Exodus 3:16, and of officers (shoterim) in Exodus 5:16.

Mention is also made of seventy elders in Exodus 24:1. Frequent mention is made in Scripture of the number seventy — a number which is composed of the two sacred numbers seven and ten — the former being the seal of the covenant, and the latter probably denoting perfection. The seventy who were chosen on this occasion may have consisted of some of those who were appointed as judges at the suggestion of Jethro, but there is no evidence of their identity with any persons previously selected.