The signs of a newborn hunger
These are, in no order of priority:
How to take care of a newborn baby
Getting a full feeding in a baby's first few days of life
This seems to be pretty important, especially for babies 2 days old. (see: Babies two days old are the most troublesome) On their second day of life, they tend to cluster feed. In particular, they get really hungry and want to eat, eat, eat. Throw out all notions of "feed once every 3 hours". Feed whenever you see The signs of a newborn hunger.
If breastfeeding is not going well, it's totally, like 100% ok to supplement with formula. Nobody ever said adding formula on top of breastfeeding was a bad idea. (BUT! Keep in mind that one should, in general, prioritize breast milk over formula milk.) If it gives the baby a full tummy so that she can go to sleep till the next feeding (typically in 3 hours from the start of the first feeding), be all for it.
How to decide how much to supplement? First off, we need an upper bound on the amount. 20 mL is what pre-made liquid formula comes in, so that's a good prior to start with. Look at her reaction. Babies that are 2 days old need (on avrage) 7-10 mL of milk, including the colostrum produced by the mother, but because during the cluster feeding phase an infant just wants to be fed, look out for The signs of a newborn hunger. (I personally find smacking of lips to be the most obvious to look for.)
Do not confuse the act of filling up a baby on the 2nd day cluster feeding phase with The goal after the first few weeks of a newborn life is full feeding each meal.
If there are signs of hunger, don't hesitate to top off with more pre-made formula. A good rule of thumb: 5 mL at a time. Don't do more. Roughly keep track of the total volume being fed; 15 mL feels like a good upper-bound for the first few days of life.
see also: How to feed a baby with a syringe
If baby is not completely full after each meal, this may extend the period during which she has issues with feeding.