It consists of pairs of rak'ahs, but if one fears morning is near, he should make it an odd number by praying one rak'ah.
Sahih Muslim Book 6, Hadith 178
← Previous Hadith Prayer during the night should consist of pairs of rak'ahs, but if one of you fears morning is near, he should pray one rak'ah which will make his prayer an odd number for him.

Sahih Muslim Book 6, Hadith 177 Next Hadith → A man stood up and said. Messenger of Allah, how is the night prayer? The Messenger of Allah (ﷺ) said: The night prayer consists of pair, but if you apprehend the rise of dawn, make it odd number by observing one rak'ah.

Sahih Muslim Book 6, Hadith 179