She has released more, she's had tonnes of non-studio projects that have added greatly to the bulk of sales.
Other 90s artists didn't have all these albums.
You can be sure if Janet released as frequently in the 80s and 90s as madonna,, more greatest hits, live albums, remix albums, soundtracks....she would have sold more than Madge.
But of course Janet kept it classy with solid studio albums only every few years, just 2 remix albums and just 2 GHs in her career