Hello everyone ? To those on mixed feeding, could you please tell me s…

Hello everyone ? To those on mixed feeding, could you please tell me something? My baby is 2 months old. I'm introducing formula once a day to give him when I need to leave for errands. Will he still need water with this? I'm thinking about how much water to give considering that it's only formula once a day. The rest of the time is breastfeeding. Please share your experience.