Koi no Tsubomi was actually certified for 2,250,000 digital downloads.
For comparison: Flavor of Life - ballad version- by Utada, which is at #12 in that recochoku 15 years ranking, was certified for 2,750,000 digital downloads.
For the Top 10 of songs you do need around 3 million and more - which sadly isn't possibly today anymore to achieve.
Btw. here's Kuu top Karaoke songs for 2016 by Joysound:
https://www.joysound.com/web/search/artist/5289/ranking
Top 10 for 2016:
01. Ai no Uta
02. Cutie Honey
03. Moon Crying
04. you
05. Butterfly
06. Suki de
07. Saigo no Ame
08. Koi no Tsubomi
09. Someday
10. real Emotion
They also have an age and gender graphic of people singing her songs (so basically also a graphic representing her fans):
Most of her fans are middle aged - but she does have a significant amount of fans in all age groups.
That's also what I've observed at her concerts.
Btw. she mostly has female fans, but the male fans she has are some very dedicated groups of males from my experience.
Also, Ai no Uta is the #46 most sung song for 2016 in DAM's Karaoke ranking:
http://www.jiji.com/sp/article?k=000...00014607&g=prt