Number of Rak`aat in Tahajjud (Ahadith 937 - 940)


First, I'd like to congratulate you for witnessing yet another Ramadan. May Allah help us derive maximum benefit and barakah from this month. And may He accept our negligible efforts.
On the Authority of Muaadh Ibn Jamal (may Allah be pleased with him) the Prophet, peace be upon him, said,
Shall I not inform you of the gates of goodness? [They are] fasting [which] is a shield, charity [which] extenguishes the sins like water extinguishes a fire and the prayer of a man in the depths of the night.
[Recorded by al-Tirmidhi]
Moving on to today's Hadith(s)..

Volume 2, Book 21, Number 238:
Narrated 'Abdullah bin 'Umar:
A man said, "O Allah's Apostle! How is the prayer of the night?" He said, "Two Rakat followed by two Rakat and so on, and when you apprehend the approaching dawn, offer one Raka as Witr."

Volume 2, Book 21, Number 239:
Narrated Ibn Abbas:
The prayer of the Prophet used to be of thirteen Rakat, i.e. of the night prayer.

Volume 2, Book 21, Number 240:
Narrated Masruq:
I asked Aisha about the night prayer of Allah's Apostle and she said, "It was seven, nine or eleven Rakat besides the two Rakat of the Fajr prayer (i.e. Sunna)."

Volume 2, Book 21, Number 241:
Narrated 'Aisha,
The Prophet (p.b.u.h) used to offer thirteen Rakat of the night prayer and that included the Witr and two Rakat (Sunna) of the Fajr prayer.

Prophet (SAW) would usually offer eleven rak`aat for Tahajjud including the witr. But it's not necessary to offer a specific number of rak`aat. All you have to make sure is that you pray in this format: 2+2+...[+1 witr].
Read more here.