A lot. Mario is just in too many different games across too many different platforms to even attempt to play or beat them all. I can't even remember all of the Mario games I've played. I can say that I skipped every Mario Golf, all of the Mario Parties except DS, Wii, and one of the N64 titles, all of the Paper Mario games except the first, Partners in Time on the DS (but I will order this, soon!).
Anyhoo, all of the Mario Karts, all of the 2D platformers, all of the 3D titles, Mario RPG, and some other spinoffs. I don't have a billion dollars to spend on Mario games.