Why did Moses select 70 elders?

16 So the Lord said to Moses, ‘Gather for me seventy of the elders of Israel, whom you know to be the elders of the people and officers over them; bring them to the tent of meeting, and have them take their place there with you. 17I will come down and talk with you there; and I will take some of the spirit that is on you and put it on them; and they shall bear the burden of the people along with you so that you will not bear it all by yourself. 18And say to the people: Consecrate yourselves for tomorrow, and you shall eat meat; for you have wailed in the hearing of the Lord, saying, “If only we had meat to eat! Surely it was better for us in Egypt.” Therefore the Lord will give you meat, and you shall eat. 19You shall eat not only one day, or two days, or five days, or ten days, or twenty days, 20but for a whole month—until it comes out of your nostrils and becomes loathsome to you—because you have rejected the Lord who is among you, and have wailed before him, saying, “Why did we ever leave Egypt?” ’ 21But Moses said, ‘The people I am with number six hundred thousand on foot; and you say, “I will give them meat, that they may eat for a whole month”! 22Are there enough flocks and herds to slaughter for them? Are there enough fish in the sea to catch for them?’ 23The Lord said to Moses, ‘Is the Lord’s power limited?* Now you shall see whether my word will come true for you or not.’