The size of a feed is very baby-dependent. A 4 month old will drink anything from 800ml to 1-and-a-bit-litres of BM a day. But some will have 5 big feeds and some will have 10 little ones and some will have a mixture of both.
How you freeze depends on how hard it is for you to express (and therefore how much "wasting" is a problem). And also how willing your baby is to wait while the second instalment of their milk is defrosted! It also changes once solids are involved and you can put the leftover milk into cereal or dinner or whatever...