How do we know what is the right amount of milk for baby?

Hi all, my baby boy is 19 days old. He need to be feed around every 3 hours. He drink breast milk and most of time top up with formula milk. Total amount ranging from 90ml to 150ml everytime depending on how hungry and if he keep crying. It got me and my wife worried that sometimes he drink a lot and vomit milk. We tried to burp him but not successfully always.