Breastfeeding 101

Hi! First time mom here with no clue what pumping entails...I'm adding things to my registry and there are 5oz and 8oz bottles and bags and sometimes in the reviews people complain the bags don't store exactly the oz they want.
This might be a stupid question but why exactly do you need to measure how much milk you pump and freeze? Can you just feed on demand and feed until baby no longer takes? 
I'm not quite sure how I should know how much to feed and at what intervals!
I'm going to be a working mom and will need to rely on pumping once baby arrives so could use any advice about what's important and things to note. Thanks in advance.