So, Oricon compiled a list of total sales for the Heisei period (last 30 years).
Singles, albums, DVD and Blu-Ray sales are counted.
Here it is again in printed form, looking a bit better readable in design:
Kuu is at #41 for all artists.
If you only look at female solos, it looks like this:
01. Ayumi Hamasaki
02. Namie Amuro
03. Hikaru Utada
04. ZARD
05. Yumi Matsutoya
06. Maki Ohguro
07. MISIA
08. Mai Kuraki
09. Miki Imai
10. Koda Kumi
11. Mariah Carey
The list consists of 11 female solos, 8 male solos, 18 bands and 13 groups/duos of any sort.
Oricon also released several statistics - one being an overview of the development of physical sales over those 30 years. Physical sales peaked in the mid-1990's and were falling pretty gradually from there.
If we will ever get a list with physical and digital sales combined, it will look a bit different than this one with only physical sales.