During the time of `Umar ibn Al-Khattab, the Tarawih prayer was commonly performed in 20 Rak`ahs, with some narrations mentioning 21 or 23 including Witr. However, the number of Rak`ahs in Tarawih remains flexible, as there is no fixed limit in Islamic teachings.

Witr prayer is a confirmed sunnah according to the majority of scholars. Whoever omits to pray Witr deliberately is a bad man, whose testimony should not be accepted.

There is no basis in the Sunnah for saying du’a between one salam and the other during Tarawih prayer by saying “Subhanaka Rabbana wa bi hamdika Allahumma ighfir lana.”

Pray Witr with the Imam, then you can pray Tahajjud after that and pray whatever Allah wills for you, two by two Rak`ahs, without repeating Witr. Another option is delaying Witr until the end of the night.

There is nothing wrong with repeating the same Surah in the Tarawih prayer, or in any other prayer. A person may recite a Surah in the first Rak`ah and repeat the same Surah in the second.