Formula feeding rules??

Okay so I'm a first time mom, my son is 3 weeks old. I was breastfeeding up until 2 days ago I started giving him formula, the doctor said I have to limit the formula feeding.?? So just wondering what are the rules n stuff for that because I don't want to over feed my baby or something. Like how many ounces each feeding n how many times do I feed him for how long in how many hours etc. Thanks.!