After praying ‘Isha and after sleeping, the time that you wake up is the time of Tahajjud, however, it is better to pray in the last third of the night. Tahujjud is Sunnah and is prayed without the intention of Sunnah and is at least two Rakats and most is eight Rakats [Fatahul Qadir, Alamgiri].
1) In a Nafl Salaah during the day it is Makrooh to pray it more that four Rakats without performing Salaam and during the night it is Makrooh to pray it more than eight Rakats without performing salaam. Whether it is be day or night it is better to perform Salaam at the maximum of four Rakats [Durr-e-Mukhtar].
2) If you have made the intention of praying more than two Rakats of Nafl then you must perform Qaidah between every two Rakats. Notice: To pray more than two Rakats of Nafl in one Salaam will make the rules rather difficult to observe and that is why it is better to pray two, two Rakats.
Salaah of Tahajjud