The answer is two.
About 90 percent of mother's milk is water. A baby drinks up to one liter of milk per day. The simple math says it is important for a mother to take almost one liter of water for the milk and one more for her body to be hydrated. This is 2 liters per day in total. Of course, the more is the better, so drinking up to 3 liters will help the mother to stay hydrated.