So Britney is the 6th best selling female worldwide, 6th in the United States, 5th in Canada & 6th in Europe.
With more than 10 million albums sold across Asia, she's without a doubt among the Top 10 biggest international females in the continent. I don't know how she ranks in Australia but I wouldn't be surprised if she's also in the Top 10. Off the top of my head, Madonna, Mariah, P!nk, Whitney & Celine have outsold her there.
She's also at least Top 5 in Latin America. Her sales are massive there. I think she's even outsold Mariah. Sales in Africa are pretty much unknown but she's sold 400,000+ albums in South Africa.
I think she might be Top 10 for females everywhere sans Africa.