Depending on the amount of times that you have to nurse, they would normally, my guess take in about 4oz. You could fill up as many btls. as you think that his grandparents are going to need and go from there. Even making up a few more. I am assuming at this age that he is nursing about every 3-4hrs. So say you fill up an ice cube tray, using two to three cubes I would think that would make up about 4oz. I believe every cube is about 2oz. Measure with water and pour into tray to see how much water it holds first. Then take it from their. Even if they have more in the btl. it is better than having less and having a fussy baby. You can even send over the frozen tray as they are going to have to warm the btl. anyways, thereby giving them enough of what the little one needs. Good luck and I hope you have a wonderful evening. Knowing that you have wonderful people taking care of your little one takes so much stress off of you. Take care.