There seem to be two basic schools of thought on photography-- take as many shots as you can, or think about what you're doing and shoot after you've composed the shot.
Quite so, but I would suggest that this has nothing to do with the percentage of keepers. Just how good your starting set is.
Of your 500 photographs I bet that if you adjusted your standards and had to choose a portfolio of 10 you wouldn't find it hard to do. Or do you really have 100 shots from that trip that are equally good with nothing that stands out as better than the rest?