I think it takes something on the order of 2+ million gallons of water to frack a single well, once. Some wells need multiple fracks.

I will say, though, that in North Dakota, they get a significant amount of their fracking water from Lake Sakakawea, i.e. Missouri River. The amount of water used for fracking is a rounding error compared to how big that lake is. Might be more of a concern in more arid areas, though.