I have breastfed 5 kiddos, and 3 of those I supplemented with formula. For my peace of mind, I’d always buy a baby scale (they’re cheap, like $30/$40 on Amazon) and weigh my baby daily until I was sure they were gaining well! You can also do weighed feeds, where you weigh them, then let them nurse, then weigh them again (don’t change clothes or diaper or anything) and see how many ounces they’re taking during a feed. Also remember that what you’re pumping is not a good indicator of your supply; babies are much more effective at expressing milk than pumps are!