Billboard just updated here best selling singles (digital and Physical)
Digital:
3,298,000, "Womanizer"
2,966,000, "Circus" (it almost reaches the 3 million)
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"
1,255,000, "If U Seek Amy"
10 singles have passed 1 million in digital sales
Physical:
1,412,000, "...Baby One More Time" (1998) (511,000 downloads)
778,000, "From the Bottom of My Broken Heart"(2000) (33,000)
415,000, "Stronger" (2001) (270,000)
60,000, "Me Against the Music" (featuring Madonna) (2003) (281,000)
57,000, "Oops!...I Did It Again" (2000) (465,000)
I'm impressed with her digital sales tbh, even though she peaked before the digital era.
Source:
http://www.billboard.com/#/column/chart ... ost_recent