I'm not going to provide links, several are currently available and others are welcome to add them. Alyse Ogletree donated enough milk to set a Guiness world record. I had to search for a few days to find out she produced an average of 130 ounces per day. I have difficulty picturing a gallon of milk per day, but she did it. She does not have any unusual problems, just overproduces massive amounts of milk after giving birth. She has 2 children and has been a gestational surrogate once, donating milk after each birth.

Elizabeth Anderson Sierra produces more with up to 7 quarts per day (225 ounces per wiki). She has been diagnosed with hyperlactation which means she can't stop producing milk.