All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Observing the night prayer brings a Muslim closer to His Lord. Allah, The Exalted, has praised those who are keen on performing this form of worship in the following verse : "They were in the habit of sleeping but little by night"{Adh-Dhariyat/51}. Moreover, the Prophetic Tradition has confirmed the greatness of this act for the Prophet(PBUH) said, "Hold fast to Qiyam al-Lail, for it is the practice of the righteous before you, and indeed Qiyam al-Lail is a means of nearness to Allah, a means of prevention from sin, an expiation for bad deeds, and a barrier for the body against disease.”{At-Tirmizi}. A'isha, umm al-muminin said, "The Messenger of Allah, may Allah bless him and grant him peace, used to pray thirteen rakas in the night and then would pray two rakas when he heard the adhan for the subh prayer."{Bukhari}.
There is no harm in performing more than eleven Rakhas during the night since the Prophet(PBUH) referred to night prayer in general, as indicated in the following Hadith, "Salat during the night should consist of pairs but if you fear that morning is near, then pray one Rak'ah as Witr."{Agreed upon}. It is evident from this Hadith that the Prophet(PBUH) didn`t specify a certain number of Rakhas for the night prayer. In addition, the night prayer is among the absolute supererogatory forms of worship. Al-Khateeb Ash-Shirbini said, " Absolute supererogatory forms of worship don`t have a specific time, reason or number of Rakhas."{Moghni Al-Mohtajj 1/346}.
In conclusion, there is no specific number of Rakhas for the night prayer, and a Muslim may offer as many as he likes. And Allah knows best.