Imam Kasani Hanafi (May Allah be well pleased with him) writes about 20 Raka’at of Taraweeh:

 Translation: There are 20 Raka’at of Taraweeh.  This is the opinion of the majority of scholars and this is the authentic one because Hadhrat Umar (May Allah be well pleased with him) gathered the Sahabah under the leadership (Imamat) of Hadhrat Ubai bin Ka‘ab (May Allah be well pleased with him), and he led them in 20 Raka’at every night and none of the Sahabah denied this (or objected to this), in this manner, the consensus of the Sahabah on this issue was achieved.

 (Bada‘i Us Sana‘i, Vol 1, Kitab Us Salaat, Pg No: 644)

Allama Ibn Taimiya’s clarification - The emigrant Sahabah and the natives of Madina, all agreed to 20 Raka’at of Taraweeh

Allama Ibn Taimiya writes in his Majmu’ Al Fatawa:

Translation: It is well established that Hadhrat Ubai bin Ka‘ab (May Allah be well pleased with him) used to lead the people in 20 Raka’at of Taraweeh in the month of Ramadhan and apart from that would lead them in 3 Raka’at of Witr.  Thus, many of the ‘Ulema say that this is a Sunnah because Hadhrat Ubai bin Ka‘ab did this in the presence of many of the Muhajirin (those who migrated from Macca to Madina) and Ansar (The natives of Madina) Sahabah and none of them opposed or denied this.

(Majmu Al Fatawa, Vol 23, Pg No: 112)

Allama Ibn Tamiya also writes:

Translation: 20 Raka’at of Taraweeh is the practice of the majority of the Muslims.  (Majmu’ Al Fatawa, Vol. 23, Pg. No. 112)

Allama Sauqi writes in Mausu‘a Fiqhiya, the encyclopedia of Fiqh, which was published the Auqaf ministry of Kuwait:

Translation: The Sahabah and the Tabie'n have offered 20 Raka’at of Taraweeh.

(Al Mausuatul Fiqhiya Al Kuwaitiya, Published by the Government of Kuwait, Vol 27, Pg No: 141/142)

It is mentioned in Al Fiqh Al Islami Wal Adilla under the Hanafi school of Fiqh:

 Translation: Taraweeh is a Sunnah and there are 20 Raka’at of it, which are offered in units of 2 Raka’at.  Witr will be offered after that.  Apart from Ramadhan, Witr cannot be offered as a congregational prayer.

 The practice of Hadhrat Umar (May Allah be well pleased with him) is the evidence of 20 Raka’at of Taraweeh as Imam Muslim (May Allah shower His mercy on him) has clarified this in his Sahih that Hadhrat Umar (May Allah be well pleased with him) gathered the Sahabah in the Masjd Nabvi to offer 20 Raka’at of Taraweeh and the Sahabah also agreed with him on this and even after the Caliphs, nobody objected to this.

 (Al Fiqh Al Islami Wal Adilla, Al Juz Ath Thaani, Al Faslith Thamin, An Nawafil ‘Indal Hanafiyya)