All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The number of night prayers is unlimited, so a person can pray whatever Allaah enables him to pray. The Prophet said: "The night prayer is in twos, and if anyone of you fears the break of dawn, he has to pray one Raka'h as Witr to end his night prayer." [Al-Bukhari and Muslim] Nonetheless, if a person prays only 11 Raka's or 13 Raka's, then some scholars stated that this is better as the Prophet prayed both ways. These are the legislated forms of night prayer and we have clarified what is best. So if you pray one way or another and wanted to do that at the beginning of the night and at the last part of it, then you have to divide the number of Raka'h between these two times. It is, however, desirable to pray Witr at the last part of it as the Prophet said: "Your last prayer of the night should be Witr." [Al-Bukhari and Muslim]
Allaah knows best.