depends on lots of things, the breed, the age of the cow, where she is in lactation cycle, her nutrition level and body condition, her genetics, and if all 4 quarters of her udder are producing. cows may produce anywhere between 3 and more than 20 gallons per day.
the quality of the milk and amount of butterfat and milk solids in the milk will vary too, by age, breed, condition, lactation cycle...
so probably the better approach is to decide how much milk you need, and what you are intending to use it for (drinking, butter making, cheese production, etc.) and then decide what sort of cow to get. or maybe decide to get goats instead.