The objection that the natural resources of the Sinai desert could never have supported two million people or more for a period of forty years' wandering is absolutely valid. But it completely overlooks what the Pentateuch makes abundantly clear: Israel did not receive its food and drink from the ordinary natural resources of the Sinai terrain. This multitude was said to have been supplied in a miraculous way with manna from the sky and water from the cloven rock, all during the journey through the wilderness. The God who led the Israelites in the pillar of fire and the cloud was the one who supplied them with their nourishment by way of a supernatural intervention on their behalf.
–Dictionary of Bible Difficulties