Billboard: Top 10 of Britney´s singles sales

3,298,000, "Womanizer"
2,966,000, "Circus"
2,638,000, "Till the World Ends"
2,225,000, "3"
1,815,000, "Toxic"
1,770,000, "Piece of Me"
1,713,000, "Gimme More"
1,592,000, "I Wanna Go"
1,520,000, "Hold It Against Me"
1,412,000, "...Baby One More Time"

And in The U.K. she was named 26th best singer singles sales with 6,2 millions, in 2009 she has 5,5 millions.

Source: billboard