I concur with others that the total number of gears is less important than the range of gears. My mid-1980s Miyata 1000 had 18 gears, and it was fine for everything, except long climbs. The problem was that I had biased the gearing toward the high end. I had set up the bicycle to more easily bomb down hills rather than climb them!
After participating on this online forum for the past six years, I have the impression that I am in the minority that believes in going as low as physically possible when selecting touring bicycle gearing. My lowest gear is currently 34/22, and I use it regularly. If I were starting again, I would choose 34/18 or 34/20.
I know someone who has 34/20 on his mountain bike, and I have seen that it is an eminently usable gear: I have seen him ride up extremely steep dirt paths that I walk up.