How many Israelites came out of Egypt and how much water would they need?

A few million Israelites came out of Egypt, accompanied by all their livestock. Given that massive population and the animals, they would need a tremendous amount of water just to satisfy their thirst as they moved through the hot desert wilderness.

Les explains that Israel had come out of Egypt and reached the oasis of Elim, but they could not stay there; they had to move on toward Sinai. They left Egypt in April, and by the fifteenth day of the second month—now May—they were in the wilderness of Zin, where the desert was getting hot. Les emphasizes that we should not picture a few families, hundreds, or even a few thousand people. There were a few million Israelites, and with all their livestock, it would take a tremendous amount of water just to satisfy their thirst. That pressing need set the stage for their complaint to Moses and Aaron.

Exodus 16:1

And they took their journey from Elim, and all the congregation of the children of Israel came unto the wilderness of Sin, which is between Elim and Sinai, on the fifteenth day of the second month after their departing out of the land of Egypt.

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Why did the Israelites need water at this point in the wilderness?

The Israelites needed water because they had moved from Elim into the wilderness of Zin, where the desert heat was increasing. With a few million people and their livestock, they required a tremendous amount of water to satisfy their thirst.