How much a baby needs has more to do with their weight than with their age. It also depends on the calorie content of your milk.
For the first six months, a baby (not preemie) needs approximately 50 calories per day per pound. So a 6 pound baby would need 300 calories, or roughly 15 ounces. This would be approximately 2 ounces per feeding (assuming your breastmilk has 20 calories per ounce), 8 times a day. A full-term newborn weighing 9.5 pounds would need about 24 ounces.
Preemies need more than this per pound of body weight to help them catch up. I think the doctors' goal at our NICU was around 60 calories per pound up to the baby's due date. When preemies are fed formula, it is often fortified to have 22-24, or even more, calories per ounce. If growth is a concern and you are bottle feeding, talk to your pediatrician about fortifying.
If you are breastfeeding, however, I wouldn't worry about all of this! I would just concentrate on making sure your baby is having plenty of wet diapers and is growing at least 1/4-1/2 pound a week.