Hi SyL,
I believe you would receive more replies to your question if you started a new thread with a different heading...
I believe the breasts perform a miraculous function. They produce milk according to supply and demand: the more your baby sucks (providing he/she has latched on properly), the more milk he/she gets. Women can provide enough milk for twins or even triplets, because their breasts are being stimulated two or three times as often. It's a shame that many women lose confidence in their body's ability to provide for their babies, because once formula supplements are introduced, this reduces the baby's time spent stimulating the breasts to produce more, which makes it difficult for your breasts to catch up. So if you believe you don't have enough milk and need to supplement, it can be a self-fulfilling prophecy.
Like many women, I have sometimes felt that my baby wants more milk, as they are learning to breastfeed and during growth spurts, but this has been temporary - you have to trust your body and wait for it to respond to increases in demand. If it does not and your baby is not gaining enough weight, there are experts you can turn to such as La Leche League or lactation consultants who can help your individual situation.
It is also helpful to go along to a LLL meeting while you are pregnant, to become more informed about breastfeeding.