I used 9 5oz bottles and 6 9oz. Since we had 8 feedings in 24 hours when we came home and for the first month or so, we still had that extra one just in case. As for the larger ones, it was convenient to to have 6 since we still had an extra. I agree that it's useless to go crazy and buy too many since you'll just accumulate dirty bottles and get discouraged!
I also used bottles exclusively (breastfeeding was a failure...) and made them for 24 hours. I got a large plastic Tupperware bottle with a lid like you'd use for juice and mixed the formula in the premeasured water. That way, I then only had to pour it in the bottles. (no powder in teets!) We also got into the habit of measuring the formula in a regular bowl and covered it until we needed it (1-2 hours later) and did the same with the water. Once boiled and cooled we put in in the pitcher. When you're tired, it's great to know that is done ahead of time. We prepared the milk in the late afternoon or early evening so it was easy to get that done every day. We knew we'd be home at that time and also didn't have to do this too late in the evening or first thing at 5 am!