You actually do have some flexibility, but generally mashes below 1.0 qt/lb are so thick they are difficult to work with. At the other end if you get the mash too thin you will not have a sufficient concentration of enzymes and your mash conversion will take a very long time.
Most sources recommend 1.25-2 qt, though traditional decoction mashes often use thinner mashes of up to 3 qt/lb.
Brad