Answer
(Fatwa: 1082/1082=J)
The taraweeh prayer is 20 rakat sunnah. For details, please see: Risala Taraweeh by Hazrat Maulana Habibur Rahman Azami. It is not right to say that the narration of 20 rakat does not exist, rather it is wrong to understand it only 8 rakat. According to all the imams taraweeh is 20 rakat. This is confirmed and agreed upon and the Ummah acted accordingly ever and everywhere. For details, see: Babut Taraweeh, Fatawa Darul Uloom Deoband, vol 4. There are many other books written on the topic, you should study them:
ق فی الدر المختار: التراویح و ھی عشرون رکعۃ و ھو قول الجمھور و علیہ عمل الناس شرقاً و غرباً الخ، وقد ثبت علیکم بسنتی و سنۃ الخلفاء الراشدین المھدیین عضوا علیھا بالنواجذ کما رواہ ابوداود۔
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband