Assuming you are talking about a typical 5 gallon batch of moonshine, here is what you can expect:
5 gallons of mash will yield you approximately 3.5-4 gallons of moonshine. This range can vary depending on the recipe used, as well as the efficiency of your distillation setup. For example, a higher gravity wash (one with more fermentables) will typically yield less moonshine due to the loss of alcohol during fermentation. Similarly, if your distillation setup is not very efficient, you may also end up losing more alcohol and thus yielding less moonshine.
Generally speaking though, you can expect to get around 3.5-4 gallons of shine from 5 gallons of mash.