Breastfeeding and going back to work

I go back to work Jan 6th so I just started pumping to prepare. I don't seem to get more than 3oz per pump session. How much milk do you prepare for a work day? How much breast milk do your babies eat at each feeding?

Baby is 3 months old, I'm exclusively breastfeeding but he will take a bottle I've pumped if I've had a drink or my husband is alone with him. When he does take a bottle, he seems to chug 3oz like it's nothing. He will whine when it's done like he wants more but will be fine a few min later. I think he overeats when he breastfeeds because he spits up a lot after every feeding. So I don't know what the right amount of milk is to pack for him. Do you think 6, 3oz bottles will be enough for a 9 hour work day? I plan on feeding him right when I drop him off. It's my co-workers mom that will be watching him. I think I'll just bring a few extra frozen milks just in case.