Ruling on only reciting Soorat al-Ikhlaas in tahajjud prayers, and repeating it a specific number of times in each rak‘ah

There is no specific way to pray tahajjud or offer
(voluntary) prayers at night, in terms of what the worshipper should recite
after al-Faatihah in each rak‘ah. So the Muslim should pray two rak‘ahs by
two, reciting whatever he is able to of the Holy Qur’an, then he should pray
Witr after that. Various ways of praying at night have been narrated in the
Sunnah, which we have discussed in the answer to question no.
46544. 

Praying tahajjud with eleven or twelve rak‘ahs, reciting
Soorat al-Ikhlaas twelve times in the first rak‘ah, then reducing the number
of times it is recited in each rak‘ah until you recite it just once in the
final rak‘ah – as described in the question, or in a similar manner – is an
innovation and is contrary to the Sunnah. 

The scholars of the Standing Committee for Issuing Fatwas
said: 

The night prayers are to be offered two rak‘ahs by two, then
if you fear that dawn is approaching, pray Witr with one rak‘ah. What the
Prophet (blessings and peace of Allah be upon him) usually did when praying
qiyaam al-layl was eleven rak‘ahs, but if anyone does more or less than
that, it does not matter. 

End quote from Fataawa al-Lajnah ad-Daa’imah (7/181). 

They also said: 

There are no specific soorahs of the Qur’an to be recited in
the night prayers; rather the individual may recite whatever he is able to
of the Quran. 

End quote from Fataawa al-Lajnah (6/103) 

Shaykh Ibn Baaz (may Allah have mercy on him) said: 

Qiyaam al-layl (offering voluntary prayers at night) is a
confirmed Sunnah, whether that is done at the beginning of the night, in the
middle or at the end, although the end is preferable, and the last third of
the night is best, unless that is too difficult. In that case one may pray
Witr at the beginning of the night, with one, three, five, seven or more
rak‘ahs, saying the tasleem after each two rak‘ahs, praying two rak‘ahs by
two, trying his best to recite with deliberation, and praying Witr with one
rak‘ah. There are no specific soorahs to be recited; he may recite whatever
he is able to, from the beginning of the Quran, from the middle of the
Quran, or from the end; or he may recite in order to finish the Qur’an (khatmah),
starting from the beginning of the Quran until the end, then repeating that.
It is all good, and there is nothing specific that is required in that
regard. 

End quote from Fataawa Noor ‘ala ad-Darb (10/25) 

And Allah knows best.