Depends on what you consider an adventure game. RPGs are very adventure-ish, and a lot of hack-and-slashes have some adventure aspects... If you mean the old Lucasarts-style adventure games, then that's a whole different beast.
At any rate, there's not an enormous amount of games on it to begin with, but a decently large percentage could be considered "adventure games" in some way or another. There are almost no "pure" adventure games anymore, though.
Socran's Razor: "Never attribute to massive stupidity by someone else, what could be explained by a tiny bit of stupidity on your own part."