Khatib Ash-Shirbini (rahmatullah ‘alayh) states in Mughni Al-Muhtaj,

“It is 20 raka’at with 10 taslimat (saying of salams), [performed] in every night of Ramadan due to what is reported by Imam Al-Bayhaqi with a Sahih (rigorously authentic) chain: ”They used to stand in [Salah] in the time of ‘Umar ibn Al-Khattab (radiya Allahu ‘anhu) in the month of Ramadan praying 20 raka’at.” [Reported by Al-Bayhaqi in his Sunan 2/496]  It is reported by Imam Malik in his Muwatta’ as “23 raka’at” and Imam Al-Bayhaqi reconciled the two narrations saying that 3 of the raka’at were for Salat Al-Witr. [Imam Malik’s Muwatta’ chapter on Salah in Ramadan 257] Also it is reported that the Prophet Muhammad “prayed 20 raka’at with them” [Al-Bayhaqi’s Sunan 2/496] as was said by Imam Ar-Rafi’i, though Al-Bayhaqi declared the report da’if (weak).” [Mughni Al-Muhtaj Vol 1 page 389, Dar Ihya At-Turath]

Ibn Naqib Al-Misri states in his ‘Umdat As-Salik,

“It is mandub (recommended) to make tarawih, and it is 20 raka’at in every night of Ramadan, in congregation, making salams every two raka’at.” [page 158]

Therefore the relied upon verdict of the Shafi’i school is that Salat At-Tarawih is recommended to pray – not obligatory as some mistakenly believe – and that it is 20 raka’at in length.

And Allah Knows Best

Shafiifiqh.com Fatwa Dept.

For more detail on Tarawih see this fatwa.