Need help figuring out how many 4 ounce bottles to purchase

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Baby boy is not due until the end of May but I am trying to get everything ready. I’ve never formula fed before but I am doing it this time and I have no idea how many bottles I’m going to need at the moment I only have 3 4ounce bottles. But not quite sure how many more I should purchase any help from mommies, who are bottlefeeding would be much appreciated.