Yuri na Watashi to Akuma na Kanojo (?)
百合な私と悪魔な彼女(?) · ゆり な わたし と あくま な かのじょ
est. Rough estimate based on volume count, reprints and magazine — not an official figure
Overview
As a child, Yashiro Nonoyama developed an intense fear of men due to boys constantly bullying her. Now in high school, she dreams of being reunited with her first love: a girl who protected her one day from her bullies. Yashiro is in disbelief when a transfer student named Mikoto Saionji arrives who looks a lot like her first love, but she later discovers that Mikoto is actually a guy, and that he was her first love. Yashiro learns that Mikoto is rapidly turning into a girl, and the only way to counteract this change is for them to passionately make out with each other every day. (Source: Kiss Manga)
Source: AniList
External databases: MADB · AniList
How to compare editions, collected volumes and ISBNs / Sources, updates and corrections