Sore wa Boku no Ai ja nai
それは僕の愛じゃない · それ わ ぼく の あい じゃ ない
est. Rough estimate based on volume count, reprints and magazine — not an official figure
Overview
Kazunobu Haizuka, an editor at a publisher, was suddenly asked by his twin younger brother to help turn down a guy from an one-night stand. But when Kazunobu went to meet the guy, it turned out to be Yukihara, the author that Kazunobu is in charge of and his long time love!
Source: AniList
External databases: MADB · AniList
How to compare editions, collected volumes and ISBNs / Sources, updates and corrections