My 99 used to worry about this too. It really drove me mad.
Don't know why she thought (and probably still thinks) formula/cow' milk is better than mama's milk.
Now I try my best to pretend that she's singing.
Not worth to wor! Will affect milk output gar!
It is proven that there is no substitute for mama's milk, not to mention formula/cow's milk. Mama's milk contains too many good substances, which are only catered for feeding your own baby. No similarity in the substance aspect is found in two mamas' milk, which is also far too complicated to replicate. Hence, there is always a statement "Mama's milk is irreplaceable" printed on the formula milk tin.
The quantity of mama's milk depends on the need of your baby. A "mechanism" is naturally built-in between the mom and the baby when you got pregnant. For example, if the baby is born underweight, this baby may need much more mama's milk just to "compensate" his/her deficiency during the fetus stage. In this sense, this baby's mom may produce more milk.
All breast-feeding moms, no need to worry about the quantity of milk. Quality is more important! No single person like your baby has a special bonding with you mom. Your baby will "tell" you the milk quantity he/she needs.
Just to stay clam and peaceful when you are breast-feeding or pumping the milk for your baby. It is the most cheerful and wonderful thing to do for your baby!!