there's no way to do that, certifications change so much for countries. The only real #s we have is for US album sales, the rest are left questionable.
Britney: 80 millions
Christina: 33 million
Pink: 35 millions
Adele: 30 millions
Rihanna: 26 millions
Beyonce: 24 millions
Taylor: 24 millions
Kelly: 18 millions
Gaga: 23 millions
Katy: 9 millions
Kesha: 2.5 millions