Breast milk is nutritious and has antibodies which will help your baby fight infections . If she is gaining weight, it means she is drinking enough. Babies get more efficient in latching as they grow.
Some research says that exclusively breastfed babies take in an average of 25 oz (750 mL) per day between the ages of 1 month and 6 months but different babies take in different amounts of milk; a typical range of milk intakes is 19-30 oz per day (570-900 mL per day).
You can try to estimate using the following:
Estimate the number of times that baby nurses per day (24 hours). Then divide 25 oz by the number of nursings.
This gives you a “ballpark” figure for the amount of expressed milk your exclusively breastfed baby will need at one feeding.
https://kellymom.com/bf/pumpingmoms/pumping/milkcalc/
Ansu Dubey