Research shows that there are no changes in a mothers milk supply by drinking water beyond thirst.
While it is important to drink lots of water while breastfeeding, it only needs to be to meet thirst. If it helps, think in terms of how much milk you are producing and how much water you drank before having a baby. A few extra glasses of water is more than likely enough when feeding an exclusively breastfed baby; as always let your body be your guide. If you feel thirsty, drink more; if you don’t feel thirsty, you are drinking enough.