How much should I pump?

Tomorrow I go in to get my tubes tied. I have to be there at 7, surgery starts at 10, 30 min surgery and two hour recovery. My son is exclusively breast fed, obviously I won't be able to feed him. I know breast fed babies 1-6 months will take generally about 3 oz at a time. I have 10 oz total. I'm still going to pump a couple of times today. I get about 2 oz each pump. One out of each boob. I just want to have a big enough supply that way if my son spits up an entire feeding and still wants to eat, my husband has enough to do this. How much more should I pump? I've done the math but I just don't know an exact amount. All I have is best guess. The nurse I saw said I should have "A couple of feedings pumped". But that is not very specific.