How much BM to pack for 24 hours?

Jessica
I'm going to have an overnight babysitter (my mom) for my 6 month old shortly and wondering how much BM to pack. I'm not sure how much he eats since I rarely give him a bottle. It won't be for quite 24 hours but I'll drop him off in the afternoon and pick him up in the morning. Anyone have a good idea on how much to pack?