I would like to know how you can tell if your baby has 'gotten enough' of your milk, and when do you need to supplement with cereal, etc..

I have read on here some ladies asking when is enough, enough, and the answers they got was you will know when the baby has had enough...

This is sort of hard to explain:
I am bottle feeding breastmilk to my babe who is 3 1/2mo. old. He has been b.fed since birth, by bottle.
I think he must feed maybe 8ish times a day...and sometimes he will only take 3 to 4 oz., and other times, he will take maybe 6 oz.
How do I know that he is getting enough when all he 'wants' is the 3-4 or 6 oz. per feed??? (actually, in reality, I'm not sure how many times I feed him, I haven't really counted...it is hard because my mom is here, and sometimes she feeds him when I have to go out, so i don't really know how many times he is actually being fed.
I am simply estimating....
Just was wondering if he is getting enough feeds in, or getting enough at one feed. His wt. at last appt. with Dr. was 13 lbs, 4 oz. (that was at 2 and 1/2 mo.

Thanks all, for any and all answers..I appreciate it.