From the Ahadith, there is proof that Salaatul Taraweeh consists of 20 Rakaah of which 8 Rakaah make up Salaatul Layl and not Salaatul Taraweeh.
A specific point shown to us by the Ulema is that the entire Quran consists of 540 Rukus. Hazrat Uthman Ghani (radi Allahu anhu) used to perform 20 Rakaah of Taraweeh every night in Ramadaan and with one Ruku recited for each Rakaah, the entire Quran used to be completed by the 27th night of Ramadaan. Wherever Hazrat Uthman Ghani (radi Allahu anhu) had stopped to make Ruku in Salaah, that stopping point was marked in the Quran with a Ruku sign. This is proof that the Khulafa-e-Rashideen performed 20 Rakaah of Taraweeh and that we have to follow the example of the Khulafa-e-Rashideen (ridwanullahi ta’ala ajma’in).