Oh, there will be more than 10 titles to sell 20 million, I'm sure.
We have 6 now. Brain Age will definitely make it (0.8 million left), as should Mario Kart DS (1.9 million left, should make it in January 11 ish). Wii Sports Resort is a shoe-in, as is New Super Mario Bros Wii. Of course, Modern Warfare 2 will also make it.
Diamond/Pearl needs 2.5 million, and it won't get that. It should end in the 18-19 million range.
Wii Fit Plus needs 7.4 million, and it's going to get that. A new Wii Fit would be the only thing I really see stopping it.
So that's 12 titles just from the released ones. I'm sure some future titles could break the mark too. 13-15 titles breaking 20 million seems likely.
Of those 12, 7 should definitely make 25 million (Wii Sports, Wii Play, Wii Sports Resort, Mario Kart Wii, New Super Mario Bros Wii, New Super Mario Bros DS, Nintendogs) and 3 have a chance (Wii Fit plus, Brain Training, Mario Kart DS). I view it unlikely that more than 1-2 future titles will sell that much.
Before the 7th gen, 4 titles had sold 25 million. (Super Mario Bros, Pokemon Blue/Red, Tetris, Duck Hunt). This gen, 7-10 titles will sell that much.
Before the 7th gen, 7 titles had sold 20 million. (the beforementioned Pokemon Gold/Silver, San Andreas and Super Mario World). This gen, 12-15 titles will sell that much.
Before the 7th gen, 10 titles had sold 15 million (the beforementioned, Super Mario Bros 3, Super Mario Land, Pokemon Ruby/Sapphire. This gen, 16-24 titles should sell that much.
I can't really go further down than 15 million for old titles just by glancing at the numbers, as I might miss out on some multiplats.
