My LG took a month to reach her birth weight but she was having lots of dirty and wet nappies so HV said it was fine to continue with just breast feeding. She is now 8 months, never had any formula and is on the 91st centile. Just wanted to say that do not get discouraged, the first weeks/months are hard, but so rewarding.
I have been told that my baby was loosing weight after 3 days being born, they told me i was not producing enough, and to supplement with formula, I was all against, but I tried once a day, only for a month, then my baby put good weight, and never again formula, I thing consuming cashews and pork chops help to increase my milk supply, my babygirl is now 17 month old, still breastfeeding, and very good weight, I believe it help her not just to comfort but to fight viral infection and heal faster.
So often with supply the more you feed, the more you produce. I would just put baby on the breast as much as possible. I had a supply issue in the beginning just combo fed for 3 days because baby had jaundice and we wanted it to clear asap and then my lactation consultant said to stop the formula top ups. Baby was at birth weight in 5 days so definitely recommend baby on the breast a lot. You can also try to pump after feeding session for short periods no longer than 5 minutes but I found that a bit too tiring while caring for a newborn