Natsu ga, Bokura no Sekai wo Miteita
夏が、僕らの世界を見ていた · なつ が ぼくら の せかい お みて いた
est. Rough estimate based on volume count, reprints and magazine — not an official figure
Overview
Note: Serialized under the title "Kimi no Sekai" (キミノ世界).
Source: AniList
External databases: MADB · AniList
How to compare editions, collected volumes and ISBNs / Sources, updates and corrections