The Messenger of Allah (ﷺ) did not observe more than eleven Rak'ah (of Tahajjud prayers), be in Ramadan or any other month. First of all he would perform four Rak'ah. Ask not about their excellence and their length. He (ﷺ) would then perform four more Rak'ah; and do not ask about their excellence and their length. Then he would perform three Rak'ah (Witr prayer). ('Aishah (May Allah be pleased with her) added) I submitted: "O Messenger of Allah! Do you sleep before performing the Witr prayer?" He (ﷺ) said, "O 'Aishah! My eyes sleep but my heart does not sleep."

[Al-Bukhari and Muslim].