The people wanted meat to eat, but the burden of feeding a minimum of three million people wandering in a wilderness, was more than Moses wanted to bear.

He spoke to the Lord about the burden of leadership, and He helped Moses in this way,

“So the LORD said to Moses: ‘Gather to Me seventy men of the elders of Israel, whom you know to be the elders of the people and officers over them; bring them to the tabernacle of meeting, that they may stand there with you'” (Numbers 11.16). 

Then the Lord said that He would take of the Spirit that was upon Moses and put upon from the Spirit those seventy men, and they would share the burden with Moses (Numbers 11.17).

The Lord chose that number because it fit perfectly with what needed to be done, for Moses had been bearing a burden that required additional seventy men to shoulder!