counter for buildings, roofs, tents
Used to count buildings, roofs, tents, and similar structures. Often seen in formal or written contexts; in casual speech, 軒 (けん) is more common for houses and buildings.
この神社には三宇の社殿がある。
This shrine has three shrine buildings.
キャンプ場にテントが五宇張られていた。
Five tents were pitched at the campsite.
軒 is the standard counter for houses and buildings in everyday language. 宇 is more formal and can also count roofs, tents, and shrine buildings.
From the kanji 宇, which originally means 'eaves' or 'roof', extended to count roofed structures.