I've never seen 15 million for Sonic 1. That's probably for all the rereleases. The MegaDrive only sold about 29 million in its life, and most of that was 1992-1994. I highly doubt there were even 7 million MegaDrives on the market when Sonic launched.
I was always of the impression that Sonic 1 sold about 4.8 million, not counting pack ins, Sonic 2 sold about 6.7 million and Sonic 3 and Sonic and Knuckles both put out about 2 million, one a bit higher, the other a bit lower. Sonic CD did about 1.5 million, if you're curious.