How many ounces pumped breast milk in each bottle for my 3 month old?

I'm going back to work in two weeks and am trying to plan how I'm going to pump and bottle feed. Currently my baby drinks about 1-2 bottles a day just for practice/ so that daddy can feed him. But he's mainly just breastfed. We usually put about 3-3.5 ounces in those, but I'm not sure if that's right for him throughout the whole day. He feeds about 7 times a day. 3.25 oz x 7 is only 23 oz which doesnt seem like much. He's around 13 lbs. How much should I plan to pump/my husband to put in each bottle?