30 million Wiis have been sold in the US

"Nintendo has announced that 30 million Wii consoles have now been sold in the US.
Not bad considering it's been on sale there for less than four years.
Topping the first-party software chart is remote-bundled title Wii Play. Other big hitters include Mario Kart, Wii Fit and New Super Mario Bros. Wii.
The most popular third-party game is Guitar Hero III: Legends of Rock, followed by Mario & Sonic at the Olympic Games."
Source:  Eurogamer 11th August 2010