Umi to Futari no Enbun Noudo
海と二人の塩分濃度 · うみ と ふたり の えんぶん のうど
est. Rough estimate based on volume count, reprints and magazine — not an official figure
Overview
Ryuuta has worked a summer job in Naoki's pub for several years, falling deeper and deeper in love with him despite Naoki's long-term girlfriend. But as soon as Ryuuta decides this will be his final summer on the job, he finds out Naoki's girlfriend is gone, and Ryuuta can no longer resist telling Nao-san about his feelings.
Source: AniList
External databases: MADB · AniList
How to compare editions, collected volumes and ISBNs / Sources, updates and corrections