Nintendo thinks 4 million 3DS will sell in a month, and 15 million games.

[Viper1]

Cross-X said:

I'm interested in how 15 Million games in total can be sold from Launch games in a Month.

15 / 4 = 3.75 gamers each.  Even the current DS has a higher attach ratio than that. 

If they hit the 4 million hardware mark, they will almost certainly top the 15 million software mark.