I read through some of the replies, and just wanted to note that I really struggled to bf both my babies, and my little girl took a full MONTH to get back to her birth weight. BUT - she was happy and content and I was so THANKFUL for my pediatrician to be so patient and helpful and not FORCE me to give her formula, that by the time she was a few months old my supply had caught up and didn't need any more formula to supplement. She is now a healthy, happy, smart, beautiful 2.5 year old.
I really think baby will let you know. My son SCREAMED nonstop as an infant - he was hungry - I had no choice but to supplement significantly with formula. They'll tell ya!