I agree, buy plenty and return what you don't use. Better that than to run out mid-stream! It looks like a good estimate is 4 to 6 balloons per foot of frame. These are so cool... have fun!
A lot will depend on what kind of balloon arch you are making, what size balloons you are using and of course what size the arch will end up being. I would make sure you purchase extra ballons as whenever we make one we always pop some while making it. You can always return any unopened bags to the store. I made a post a while ago on how to make a balloon arch if you need directions.