Yeah I know. It's just overall sales/#volumes is a good way to see how popular series are in comparison to each other despite having different lengths. Those sales are quite low considering how popular Eyeshield seems to be. It's interesting.
We only have concrete numbers for Eyeshield 21 starting around 2008 which is about Vol 29.
アイシールド21
│ 初動. (日)│ 2週計| 3週計| 4週計| 5週計┃ 累計 (日数)┃ 発売日.│タイトル
│118050 (3)│188660│210964│------│------┃*,210,964 (*17)┃2008/04|アイシールド21 29
│170685 (5)│214426│230427│240167│------┃*,262,268 (166)┃2008/06|アイシールド21 30
│193271 (7)│235667│251579│------│------┃*,281,274 (105)┃2008/08|アイシールド21 31
│157911 (6)│196111│213120│------│------┃*,213,120 (*20)┃2008/11|アイシールド21 32
│170927 (7)│205635│220798│229767│------┃*,270,722 (322)┃2009/01|アイシールド21 33
│161988 (5)│212820│235464│------│------┃*,279,332 (264)┃2009/03|アイシールド21 34
│103333 (3)│183504│200971│210902│------┃*,244,240 (206)┃2009/05|アイシールド21 35
│165014 (6)│212970│229606│236565│------┃*,255,635 (111)┃2009/08|アイシールド21 36
│109645 (3)│184146│206279│------│220870┃*,228,368 (*52)┃2009/10|アイシールド21 37
I'd also say its slightly above 20 MIllion as 20 mil was up till vol 35. I wouldn't really say this is low as the series was winding down and for lots of sports series that go this long, they start winding down towards the end. Prince of Tennis peaked with a print number of 1.11 million for volume 16 and even it winded down to around 300k for its last volume, volume 42.