Yeah...I'm kind of in the same position. I SHOULD know how many he goes through, but I don't. I'm thinking 1 at night and 3-4 during the day, but I also have to take into consideration any pool time we may have, because we'll probably put a fresh diaper on him after either way.
Then there may be times when we change him because it's convenient (ie when everyone else is headed to the restroom) even though his diaper isn't as "full" as we would normally let him get to.
Usually I pack a whole sleeve (half a box) of diapers when we travel, and we go through a little over half in a week, so I'm thinking 40 diapers should cover 8 days. I'm planning on looking at the packages on my "final" shopping trip tonight, which should help me make my decision. I have it in my head that the smallest package is like 36 diapers, so if I pack 10 for the trip down, we'd have 46 total after our shopping trip, which would be almost 6 per day. So 1 for the night (he almost never needs changed in the middle of the night) and 1 every 2-3 hours during the day.