Dr. Greene's Answer
Average babies of that weight tend to take at least 28 ounces a day of breast milk. But every baby’s needs are different and individual babies need ever-changing amounts of food, influenced by their activity level and developmental phase, the air temperature and relative humidity, and perhaps by a virus they might have. So how is a parent supposed to know how much to give?
Thankfully, babies are born with a sophisticated internal mechanism for determining just how much they need to thrive. Healthy babies, given the opportunity, will tend to drink the amount they need. The times to get concerned are when they aren’t gaining weight appropriately, don’t seem satisfied after eating, or seem to be getting dehydrated (making urine less than every 8 hours would be one signal of this).
Usually, 15 pounds is an ideal weight for a 5-month-old baby girl. Unless she is unusually tall (>27 inches) or not following her growth curves, she is likely to be getting just what she needs from your breast milk (plus some extra vitamin D from sunshine or a supplement)