Our guidelines are around 20 oz in a day, but this includes milk in cereal and food, as well as yogurts, cheeses, custards etc.
I wish I could actually get my DS to take those amounts of milk. Some days he will happily drink 6, and then we have days like today where he takes none. I know he is teething so guess this could be the case too. Thankfully I can get 8oz in his cereal, and he loves his yogurts.