Up to 10 Settlers can work on a Mill at once, though due to collisions between units it is best to only use 7 per structure. Mills placed too closely together (side by side with no space between them) may cause some Villagers to "migrate" to the adjacent Mill; this bug occurs with Plantations and Rice Paddies as well.
"A mill is used to break grains down into smaller pieces making cooking and baking easier. They were historically powered by wind, water, or livestock. Grain is ground between stones, known as millstones, until it is fine enough to be used for whatever purpose is needed. Flour for bread was made in this manner."