Good advice from the 2 other posters.
Must add: always put the water in the bottle first, THEN add the Formula powder. Otherwise, it will be too "concentrated" if you put in the powder then the water.
Even when on Formula, always feed ON demand. NOT per a schedule. And still, just like when breastfeeding, when going through growth spurts, a baby will need/want more feedings... and this is 24/7. Day and night.
My son, from about 6-8 months old, took Formula in conjunction with my nursing. He had a GINORMOUS appetite, and per various things, my Ped advised to supplement him with Formula. It was fine. What I would do is nurse first, then give him a bottle... gradually, when he was about 1 year old, he just self-weaned from breast.
Each baby is different. Always ask your Pediatrician if you have questions, or need suggestions as well.
each baby will drink varying amounts.. and it is NEVER the same amount each day, because they are constantly growing or teething and like an adult, their appetites can vary. So... this is why, always feed ON demand.... he is only 3 months old.
What I would do is make a bottle that is about 6 ounces. Then see how he drinks that. If he gulps it down and finishes the bottle in 1 sitting, then you can add more... and gauge his intake/appetite that way. If he finishes the whole bottle and "seems" to want more, then certainly, make another bottle or make a bit more... maybe 2 more ounces. Then give it to him and see. That way, you will get to know his intake/appetite needs.
Try not to "under" feed. Otherwise he will not be getting enough intake and may still be hungry.
So just gauge him... and see what his typical intake is.
Some babies drink a lot, some don't.
My son at that age, took about 6 ounces, per sitting. That is after I nursed him. And he'd want to feed every 3 hours or less. And he "cluster fed" on top of that at times, during growth spurts and developmental changes.
So, that is why, feed "on demand."
All the best,
Susan