a) Aisha (Allah be pleased with her) reports that the Prophet (Peace be upon him) never prayed more than 8 rak'aats in taraweeh, so how come nobody disapproves of 20?

(b) Is it true that Umar (Allah be pleased with him) introduced it?

Ans. 22
(a) As regards the taraweeh prayer - people agree that the Sunnah of the Prophet (Peace be upon him) and the best way is 11 rak'aats. As regards any addition - then this is DISAPPROVED of and DECLARED AS A BID'AH (A bad innovation) by Shaykh al-Albani and by a few earlier scholars - that being reported from Imam Malik, Ibn ul-Arabee and as-San'aanee (see Salat-ut-taraweeh of Shaykh al-Albani).

(b) It is not true that Umar (Allah be pleased with him) either prayed or ordered 20 rak'aats. Rather he ordered Ubayy ibn Ka'b to lead the people with 11 rak'aats (al-Muwatta 1/137, with a Sahih Isnad).