There's no hard and fast rule from what I've read about formula feeding but you can definitely take cues from your baby :) if he finishes his bottle quick and smacks his lips or seems hungry, you can add more for the next feed, and if he is fidgety or easily distracted, he's probably had enough. Some guidelines say for 2-4 mth old, it's 4-6oz every 4-5hrs..some say it's 2-2.5oz per pound of body weight within a 24hr period (e.g. 5kg or 11lbs baby will take 22-27.5oz in a 24hr period. You know what your baby needs best :)
Ziee