How much 20 million sellers will Nintendo DS in your opinion.

I bet five at least. Nintendogs, NSMB, Brain Training 1 & 2 and Mario Kart.

Pokemon D/P may have a shot but with the release of Platinum I doubt. Animal Crossing is pretty difficult but with the release of the Wii version who knows.

I'm not sure if Mario Kart can do it. it's at what, 11.5 mil? so thats 8.5 mil to go selling roughly 65k / week would take 31 months, knocking off 5 months for holiday boosts could take it just a little over 2 years. Nintendogs a definite 20 mil, NSMB a definite, Brain training 1 probably, not sure about 2.

So New Super Mario Bros will be the first game ever to sell 20 million units without being bundled or having multiple versions.

NSMB, Nintendogs, and Brain Age 1 definitely will.
I'd like to see Diamon/Pearl to do it but with Platinum I don't see it happening.

Nintendogs definitely but it's a hard call. I mean the long standing games such as New Super Mario Bros, Brain Training, More Brain Training, and Pokemon Diamond/Pearl if they kept at similar rates for a couple more years would do it easy but who knows if that will stay.