4. Nicki Minaj 100 million units* (8.5 million of which are albums)
4. Mariah Carey 90 million units (66 million of which are albums)
the difference between Mariahs and Nicki's album sells are a kii
Also all these fckin CAVEATS to Nicki's number

Notes*Nicki Minaj's total includes her work with Young Money (0.5 million albums and 3 million singles). The total also includes certifications for Nicki's remixes of "Down in the DM" (award presented to Yo Gotti and Nicki Minaj) and "Dance (A$$)" (RIAA website specifies the remix).
Despite receiving credit on Billboard for both "Plain Jane" and "The Way Life Goes", it's unclear which versions the RIAA considers primary. For this reason, they have been excluded. Including those tracks, Minaj's total units would rise to 93 million.
All other remixes were either considered promotional singles ("Hello Good Morning", "All I Do Is Win", etc.) and didn't receive credit on Billboard or remain uncertified ("Till the World Ends").
Nicki Minaj, along with many other artists referred to in the ranking above, have under-certified catalogues. In addition, RIAA certifications are not entirely appropriate in expressing overall career sales.