How much breast milk for an 8 month old?

I'm going back to work soon and will be pumping. How much do/did you give your 8 month old? Mine currently feeds every 3 to 4 hours during the day (unfortunately about ever 2/3 hours at night). I'm worried I won't pump enough so I'm considering supplementing with formula but then not sure how much to decide how much to give him then. Any tips are appreciated!