How much should baby be eating?

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So I’ve been doing a lot of research and mostly everything thing I find says that breastfed babies ( I can’t speak for formula fed) but their intake of milk doesn’t really go up after a while until they eat solids then it decreases. Baby boy is 3 months old and I save 4 oz bags of milk because I feel he shouldn’t really be eating more than that but my mother in law insists he eats a bunch when she watches him. I only work 4 hour days. Today I fed him at 9:00 and left him with her at 9:20. When I went to pick him up at 2:30 she said he had eaten the whole 4 oz and then she gave him 3 oz of formula since there wasn’t enough milk. Is that too much to be eating? She seems to think soon we’ll need to feed him 8oz. Most places I’ve found have said that about 6 oz is usually the most they should be eating. I just don’t want him to be getting over fed.