The people who hold that twenty rakats taraweh is a compulsory sunnah (Sunnah Muakiddah) usually bring forth two arguments, the first being that Umar (May Allah be pleased with him) united the Muslims upon twenty rakahs behind Ubay Ibn Ka’ab and Tamim Daari. The second being that the four Imams unanimously agreed that twenty rakahs is sunnah muakiddah.
Regarding how many rakahs Umar (May Allah be pleased with him) united the people
upon, Imam San’ai says, "There is a difference of opinion in the amount that Ubay Ibn Kab read. Some say it was eleven rakahs, others say it was twenty one rakahs, others say twenty, others say twenty three and there are other opinions as well," (Subulus Salaam Vol:2 Page: 356)
So we see that the scholars don’t even agree upon how many rakahs Umar united the people upon so this can not be a proof.
Regarding the four Imams, Imam Malik (May Allah be pleased with him) narrated the following in his Muwatta, "Umar ibn al-Khattab ordered Ubayy ibn Kab and Tamim ad-Dari to watch the night in prayer with the people for eleven rakats. The reciter of the Qur'an would recite the Mi'in (a group of medium-sized surahs) until we would be leaning on our staffs from having stood so long in prayer. And we would not leave until the approach of dawn," (Muwatta Malik 6:2:4). This narration also shows that Umar united the people upon eleven rakahs, not twenty.
Regarding the Hanafi viewpoint, Imam Muhammad, the student of Imam Abu Hanifah, mentions the following narration in his Muwatta, "the prophet (peace be upon him) would not read more than eleven rakahs, neither in Ramadan or out of it (Muwatta Muhammad, Chapter of the Ramadan prayer 71:239)
Imam Muhammad then says, "We (The Hanafis) go according to all of this, there is nothing wrong in praying in the month of Ramadan, the people praying voluntarily with an Imam," (Muwatta Muhammad, Chaper of the Ramadan prayer 71:241)
This shows that the belief of Imam Abu Hanifah and his students was that Taraweh is optional. When the hadith says eleven rakahs, it could mean eight rakahs taraweh and three rakahs witr or ten rakahs taraweh and one rakah witr, Allah knows best.
So it is clear that Taraweh is optional and each person should read what he is able to, with eleven rakahs (including the witr) being the sunnat. To make twenty rakahs compulsory upon Muslims is an innovation with no basis in the Quran or Sunnah.