Kimi to Hamu Hamu Shitai no da!
きみとハムハムしたいのだ! · きみ と はむはむ したいのだ
est. Rough estimate based on volume count, reprints and magazine — not an official figure
Overview
Kamiya, who loves small animals, discovers that Matsuyuki, a kendo rival from another school, can transform into a hamster...!? Note: Includes 2 extra chapters.
Source: AniList
External databases: MADB · AniList
How to compare editions, collected volumes and ISBNs / Sources, updates and corrections