3 of the 4 Imams said 20 is Sunnah.
Imam Maalik (rh) said 33 is Sunnah.
Others said more.
Hanbalis generally say 20.
How do we explain this?
The Sunnah of the Prophet (saw) was 11 rak’ah which was related by A’aisha (ra) in Sahih Bukhari. However, in another Hadith, which is Sahih, a Sahaabi asked the Prophet (saw) about the night prayer which he (saw) replied, it’s two by two and if you fear dawn complete it with one, to the nearest meaning.
Allah knows best.
So what do we do?
The Sunnah is to pray two by two as the Prophet (saw) mentioned for the night prayer. For this reason, some of the Salaf would pray 33, Imam Ahmad (rh) would generally pray 300 rak’ah per night.
The best approach is, as Imam Ahmad (rh) mentioned, pray with the Imam and stay with him until he leaves and you get the reward for praying the entire night and Allah knows best.