It's ok. What I do know is the highest novel(not light novel) sales on oricon in 2014(and i doubt it has been overtaken). Eien no zero(kodansha ver.) did 4,383,020(5 years) at the end of 2014 and oricon stated that it was the highest in any category for a volume.
oricon article when eien no...
I don't know the biggest first week for a light novel but haruhi vol. 10 did more in its first week.
Weekly Light Novel Rankings for May 23 – May 29
*1, 341,184 *,341,184 Suzumiya Haruhi Series vol.10 Limited Edition
Source
Kimetsu vol 1 finally sold 900k. On other hand its latest volume is like ~20k below the latest hxh volume before it fell off the chart,right?
The sales are uneven, did they have special volumes for some of them?