How much to feed my newborn?

Hi, FTM here. My little one is now 5 days old, and I've been breastfeeding him for about 20-25mins per breast every feeding. He seems satisfied each time but there are occasions when I can't get him to feed on the other breast after 20min because he's really sleepy, so my confinement nanny would top up an additional 60ml of expressed breastmilk and he actually finishes the bottle. We've not done this previously at the hospital and I noticed he started to throw up milk today. Is this a sign of overfeeding? He also gets hungry quicker, and my nanny has been handing him to me for feeding every hour or so. It made me so tired that I've told her to feed him formula after 12mn but I'm feeling so guilty...