How little breastmilk is enough for providing essential benefits?

Hi everyone! I'm currently pumping about 60-70 ml (which is roughly 2-2.4 oz) of breastmilk each day due to low supply. My baby latched well initially but developed a preference for bottles after 1.5 months. I'm curious, how little breastmilk can still provide essential benefits to my baby? Also, does milk supply decrease if the baby doesn’t latch and I only pump? Lastly, I would appreciate any suggestions on ways to increase my milk supply. Thank you!