Tarawih is prayed with eleven or thirteen rak`ahs. Following the example of the Prophet (peace be upon him), this prayer should be performed two by two (every two Rak`ahs together) followed by the Witr which is one Rak`ah. However, there is no harm for a person to perform it with twenty Rak`ahs or more as the Prophet (peace be upon him) said,
Prayers at night are to be offered two by two (two Rak`ahs at a time). If any of you fears that the time of dawn is approaching, let him pray one Rak`ah as Witr.
Reported by Al-Bukhari and Muslim.
Accordingly, the Prophet did not set a limited number of Rak`ahs for this prayer. Moreover,
`Umar and the Prophet's Companions (may Allah be pleased with them) who are the most knowledgeable of the Sunnah, performed it with twenty Ra`kahs on some nights apart from Witr. (Part No. 7; Page No. 199) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.