How many oz in a feeding?

Today is my second day back at work and before now I was EBF. My guy is drinking pumped breastmilk. Yesterday he drank about 20oz (4 or 5 oz in a feeding) for my husband (plus 2 BF sessions) so I packed that much plus a little extra to take to my parents who are watching him today. My mom called around 2pm to say he was drinking 7oz at a time and she was almost out of milk! She said she offered 5oz and he was still screaming after so she gave him more and he was happy. Is this too much? I typically pump between 4-6oz in a session so I don't know how to keep up with that much!