how many ounces should baby be eating?

Hello! Baby girl is exactly 6 weeks. Right now I've been exclusively breast feeding. I'm going to start pumping once a day to have a good stash ready for when I go back to work at 8 weeks. My question is how do I know how many ounces she eats or needs?? I can't tell how many ounces she eats now obviously since she is on the boob. Also this may be silly but I'm also starting to feel really sad. I'm scared she is going to think I'm abandoning her when I go back to work. Luckily my parents and husbands parents will be watching her so that makes me feel better but do babies feel that way when we leave them? I'm scared she is going to forget me 😩😩