Actually it is madonna that sold the most
she sold like 50m more than mariah
and 32m more than celine

Mariah stopped being a pop artist/ adult contemporary artist a long time ago though. Celine immediately took over that vacancy and just stayed there. Mariah evolved to a pop/r&b artist, singing uptempos and competing with Janet and Madonna, then to r&b where she competed with Toni/Whitney while Celine just stayed where she was and delivered the same music essentially her entire life.

Mariah made a bold choice to sing what she wanted, and if that made her less global, she didn't seem to care. Despite the global-ness of Celine, it's strange that she seems to be the least respected of the top 4 females of all time - Madonna, Mariah, Whitney. Mariah was the one who reaped all the icon/legend/lifetime achievement awards after their reigns were over.

And I still don't believe those certifications from other countries. I mean, are those even official? It's easy to inflate their numbers.

And even if Celine wins because she made all those French albums before her international big break, she isn't half the artist and vocalist that Mariah is.

Celine sings adult contemporary, Mariah sings R&B so it's not like they're chasing the same market.

In fact, besides Music Box, the only thing they have in common is being top tier vocalist.