I own Sapphire, Emerald, Leaf Green, Pearl, Platinum, Soul Silver.
I used to own Blue, Silver, and Crystal.
My brother used to own Red, Gold, the Game Boy Trading Card game, Stadium 1, Stadium 2, Snap, and Pokemon Channel as well, I believe
My brother currently owns Ruby, Emerald, Fire Red, Diamond, Pearl, Platinum, Heart Gold, Colosseum, XD Gale of Darkness, and Battle Revolution.
So, I currently own six, and between my brother and myself we own 16. We've owned a total of 26 at some point.