There's a Sega Master system version of Sonic.
Then there's Sonic 1,2,3, Sonic and Knuckles on Genesis. There was also a Sonic 3d on either Sega CD or 32x... maybe both.
Then dreamcast had Sonic Adventure 1 and 2.
I'm almost sure I'm missing some games... this is just off the top of my head. Oh, and there were also at least 2 Game Gear Sonic games