Bottle Feeding

My baby eats both expressed breast milk and formula via bottle. He’s a 2 month old preemie (2 weeks adjusted). He doesn’t eat a lot in one sitting. Basically he just snacks all day (bottle feeding on demand) which makes for a lot of feedings (16 in total today) and a lot of dirty bottles.

My question is when do babies take a full bottle in one sitting? The formula tin says to give them 6 bottles a day with a certain amount but he will not eat all of this in one sitting. Also, on the other side of it, if he shows signs of hunger obviously I’m going to feed him but where he is snacking all day he won’t eat as much. Does this change eventually? Not sure how it’s supposed to work. He is getting what he is supposed to eat in a day but it’s just spread out over many many small feedings.