Tarāwīḥ is the night prayer of Ramaḍān (qiyām Ramaḍān). The Prophet ﷺ said: "Whoever stands in prayer during Ramaḍān out of faith and seeking reward will have his previous sins forgiven" (al-Bukhārī 37). He led it in the mosque for a few nights, then stopped coming out, saying: "Nothing prevented me from coming out to you but the fear that it would be made obligatory upon you" (al-Bukhārī 1129). After his death ʿUmar gathered the people behind Ubayy ibn Kaʿb (al-Bukhārī 2010), and praying it in congregation in the mosque is the established practice. As for the number, ʿĀʾishah reported that he ﷺ never exceeded eleven rakʿahs in Ramaḍān or outside it (al-Bukhārī 1147), so eleven or thirteen is a complete following of the Sunnah; yet twenty and more was practised by the Salaf and is the position of a large body of scholars, since the Prophet ﷺ set no upper limit when he said "the night prayer is two by two" (al-Bukhārī 990). This is a legitimate difference of opinion in which no one should be censured or disputed with — length, recitation and khushūʿ matter more than the count.
Q&A · Prayer
What is Tarawih prayer and how many rak'ahs should it be?
Evidence — the daleel
Sahih al-Bukhari 37 ṢaḥīḥSahih al-Bukhari 1147 ṢaḥīḥSahih al-Bukhari 2010 ṢaḥīḥSahih al-Bukhari 1129 ṢaḥīḥSahih al-Bukhari 990 Ṣaḥīḥ
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.