2006 had 2. (RRR and Red Steel.)
2007 will end up with at slew of them. Only a couple to reach within the calender year, but I don't see how that's important. (RE4:Wii, GHIII, Carnival Games, RE:UC, MySims, Sonic Secret Rings, LEGO Star Wars, RRR2, High School Musical, Cooking Mama, and Smarty Pants could all reach 1M. I'd say at least 9 of those 12 will reach 1M. Mario and Sonic doesn't really count.)
So if 2007 sees 9 million-sellers, and in 2008 hardware sales increase and the install base grows, 2008 should see far more than 9 third party million-sellers.
I'll wager there will be at least 15. Prediction: at least 15 third-party Wii games released sometime in 2008 will eventually reach 1M in sales.
That number could be 20 or more if the EA yearly franchises finally break through. A big part of the Wii's "bad third party sales" is just Madden, FIFA, Need for Speed, the Sims, and all the other yearly franchises everyone seems to hate underperforming on Wii.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.