17.
I think maybe they didn't make the original Marowak a ghost dual type, because they wanted to keep the ghost typing really special, like they did with dragons, so they only allowed one line in Kanto to have it, gastly.
Something I find curious about ghost pokemon is that according to the pokedex entries many of them were human once. The impression given is that sometimes people who die can turn into ghost pokemon. But at the same time ghost pokemon don't seem to be truly ghosts of dead people either. They seem like they are still alive, they can eat, breed, hold items, leave footprints, experience various status conditions. I know they have to go with the mechanics of the universe, but maybe they're closer to yokai than actually ghosts.