What if my baby is still hungry after breastfeeding? Should I give formula?

Switch. Switch to the other breast, or let baby suck - milk production depends on demand and you will not have the stimulation to produce more if baby is given formula instead. Even non-nutritive sucking is beneficial to baby and mother.
No! Breast milk is made of specialized fats, proteins, & carbohydrate complexes that are easy for a baby's digestive system to "break down" & process. High digestibility of breast milk enables infants to digest/absorb nutrients of breast milk more easily than those on infant formula. Because babies digest breast milk quickly & efficiently, they feed more often than babies who are fed formula.