Number + 面 + Noun (flat object)
面 (miàn) is a measure word used for flat, broad objects such as mirrors, flags, shields, walls, and drums. It is the appropriate classifier when counting or specifying these types of items in Chinese.
墙上挂着一面镜子。
Qiáng shàng guà zhe yī miàn jìngzi.
A mirror is hanging on the wall.
广场上飘着好几面旗子。
Guǎngchǎng shàng piāo zhe hǎo jǐ miàn qízi.
Several flags are fluttering in the square.
他们抬来了一面大鼓。
Tāmen tái lái le yī miàn dà gǔ.
They carried over a large drum.