counter for songs and poems
「首」 has other readings with different meanings.
neck
office; official
Obito (hereditary title, often given to powerful regional families)
head; head of a person, doll, or corpse
発表会で三首の和歌を歌いました。
I sang three songs.
秋の美しさを詠んで短歌を一首詠みました。
I composed one tanka poem.
古い詩集には有名な五首の詩があります。
There are five poems.