What is the maximum and minimum number of rak'ahs permitted for voluntary prayer in one session, and is a single rak'ah valid?
Chapter on the Times When Prayer is Prohibited
Al-Mughni
Book of Prayer
Primary text
Some scholars maintain that one must not exceed two rak'ahs for voluntary night prayers and four rak'ahs for voluntary day prayers; performing one or three rak'ahs is invalid. The primary evidence against performing a single or odd number of rak'ahs is the authenticated Prophet's saying, "The prayer of the night is two by two," and the principle that legal rulings are derived solely from the Prophet's explicit statements or the meaning thereof, which single or triple rak'ahs lack.
Supporting text
Al-Qadi stated that praying six rak'ahs in either night or day is disliked but valid. Abu Al-Khattab holds two varying opinions regarding the validity of a single rak'ah; one opinion permits it, based on a narration where 'Umar ibn al-Khattab prayed one rak'ah and stated, "It is voluntary, whoever wishes may add, and whoever wishes may decrease."