How Many Rak’ahs in Tahajjud
Question

If a man prays two rak`ahs or one rak`ah on his own, out of faith and in hope of reward, every day after ‘Isha in Ramadan, will he be regarded as having prayed qiyam in Ramadan out of faith and in hope of reward?
Praise be to Allah.Firstly:
Virtues of Qiyam al-Layl
Qiyam al-layl is a great act of worship in Ramadan and at other times, but it is more important in Ramadan. Prophet Muhammad (blessings and peace of Allah be upon him) said: “Whoever prays Qiyam in Ramadan out of faith and in hope of reward, his previous sins will be forgiven.” (Narrated by al-Bukhari (37) and Muslim (759)
How Prophet Muhammad prayed Qiyam al-layl
The best of Qiyam is that which is in accordance with the way the Prophet (blessings and peace of Allah be upon him) did it, which is praying eight rak`ahs, making them beautiful and long, then praying Witr with three rak`ahs. `Aishah (may Allah be pleased with her) said: The Prophet did not pray more, in Ramadan or at any other time, than eleven rak`ahs. He would pray four, and do not ask how beautiful and long they were. Then he would pray four, and do not ask how beautiful and long they were. Then he would pray three. (Narrated by al-Bukhari (2013) and Muslim (738)
Minimum number of rak`ahs for Tahhajud
The minimum number of rak`ahs for Qiyam is two; there is no maximum limit for it, because of the report narrated by al-Bukhari (472) and Muslim (749) from ‘Abdullah ibn ‘Umar, who said: A man asked the Prophet (blessings and peace of Allah be upon him) when he was on the pulpit: What do you say about the night prayers [Qiyam]? He said: Two [rak`ahs] by two, then when you fear that dawn is about to break, pray one [rak`ah] and that will make what you have prayed odd-numbered.”
Abu Dawud (1451) and Ibn Majah (1335) narrated that Abu Sa‘id al-Khudri and Abu Hurayrah (may Allah be pleased with them) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever wakes up at night and wakes his wife, and they pray two rak`ahs together, they will be recorded among the men and women who remember Allah much.” (Classed as sahih by al-Albani in Sahih Abi Dawud)
This hadith indicates that Qiyam al-layl may be done with two rak`ahs.
Abu Dawud (1398) narrated that ‘Abdullah ibn ‘Amr ibn al-‘As said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever prays Qiyam reciting ten verses will not be recorded as one of the heedless. Whoever prays Qiyam reciting one hundred verses will be recorded as one of the devout. Whoever prays Qiyam reciting one thousand verses will be recorded as one of the muqantirin.”
Al-Mundhiri said in at-Targhib wa't-Tarhib: “The phrase “one of the muqantirin” means: one of those for whom will be decreed a qintar [huge amount] of reward, starting from the Surah Tabarak alladhi bi yadihi’l-mulk [i.e., Surat al-Mulk] until the end of the Quran, one thousand verses. And Allah knows best.”
Some of the jurists said: “The minimum for Qiyam is eight rak`ahs.”
And some of them said that it means spending [the entire] night or most of it in prayer.
It says in ad-Durr al-Mukhtar: “The minimum for the night prayers, according to what it says in al-Jawharah, is eight [rak`ahs].”
Ibn ‘Abidin said in his commentary on ad-Durr al-Mukhtar (2/25):  “With regard to the words “The minimum for the night prayers, according to what it says in al-Jawharah, is eight [rak`ahs]”, the reason why he said “according to what it says in al-Jawharah” is because in al-Hawi al-Qudsi he said: He may pray whatever he is able to, even if it is only two [rak`ahs], but the Sunnah in [Qiyam al-layl] is to pray eight rak`ahs with four taslims. End quote. … He also said in it: This is based on the fact that the minimum the Prophet (blessings and peace of Allah be upon him) prayed in tahajjud was two rak`ahs, and the maximum he prayed was eight rak`ahs, quoting from what is mentioned in al-Mabsut by as-Sarkhasi. Then, to support his shaykh, al-Muhaqqiq ibn al-Humam, he quoted the hadiths which prove what his shaykh specified in al-Mabsut and the hadith of Abu Dawud which proves that the minimum that the Prophet (blessings and peace of Allah be upon him) prayed in tahajjud was four rak`ahs, apart from the three rak`ahs of Witr .
But towards the end, he narrated that the Prophet (blessings and peace of Allah be upon him) said: “Whoever wakes up at night and wakes his wife, and they pray two rak`ahs together, they will be recorded among the men and women who remember Allah much.” Narrated by an-Nasai, Ibn Majah, Ibn Hibban in his Sahih, and al-Hakim. Al-Mundhiri said: It is sahih according to the conditions of al-Bukhari and Muslim.”
I say: we should say that the minimum for tahajjud is two rak`ahs, the medium is four, and the maximum is eight. And Allah knows best.
In al-Fatawa al-Hindiyyah (1/112) it says: 
“Among them [the prayers] is Qiyam al-layl, as is mentioned in al-Bahr ar-Raiq. The maximum that [the Prophet (blessings and peace of Allah be upon him)] prayed in tahajjud was eight rak`ahs and  the minimum was two rak`ahs. This is what it says in Fath al-Qadir, quoting from al-Mabsut.”
In al-Fawakih al-Dawani (1/201) it says: 
“In the case of the Prophet (blessings and peace of Allah be upon him), tahajjud was obligatory, because of the report narrated by al-Bayhaqi: “There are three things which are obligatory for me and voluntary for you: tahajjud, which is Qiyam al-layl, Witr, and Duha.” The minimum of what was obligatory for the Prophet (blessings and peace of Allah be upon him) of that was two rak`ahs.”
In al-Mawsu‘ah al-Kuwaitiyyah (14/88) it says: 
“The jurists are unanimously agreed that the minimum is two brief rak`ahs, because of the report narrated by Abu Hurayrah (may Allah be pleased with him), that the Prophet (blessings and peace of Allah be upon him) said: “If one of you gets up to pray at night, let him start his prayer with two brief rak`ahs.” 
Al-‘Ayni said in ‘Umdat al-Qari (1/228): 
“Does the phrase “Whoever spends Laylat al-Qadr in prayer” mean that one is required to stay up all night, or could it be sufficient to do the minimum that may be called Qiyam (voluntary prayer at night)?
I say: The minimum is sufficient, and this is the view of some leading scholars, to the extent that it was said that the obligatory prayer of ‘Isha is sufficient to include a person as having prayed Qiyam. But what appears to be the case according to custom is that it cannot be said that someone prayed Qiyam unless he stayed up for the entire night, or most of it.”
The correct view is what we have noted above, which is that praying Qiyam may be achieved by praying two rak`ahs.
Can you pray Qiyam as one rak`ah?
There is a report which indicates that the minimum of Qiyam is one rak`ah, but it is not sahih.
It was narrated that Ibn 'Abbas (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) instructed us to pray at night, and he urged us to do that to the extent that he said: “You should offer voluntary prayer at night, even if it is only one rak`ah.” (Narrated by at-Tabarani in al-Kabir and al-Awsat)
This report was classed as weak by al-Albani in Da‘if at-Targhib wa’t-Tarhib (365).
It was narrated that Ibn 'Abbas (may Allah be pleased with him) said: I mentioned Qiyam al-layl, and some of them said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Half of it, or one third of it, or one quarter of it, the time between two milkings of a she-camel, or the time between two milkings of milking a ewe.” (Narrated by Abu Ya‘la, classed as weak by al-Albani in Da‘if at-Targhib wa't-Tarhib (364)
“the time between two milkings of a she-camel” – that was because they would milk the she-camel, then leave her for a while to allow the calf to suckle, so she would begin to produce more milk.” (Fayd al-Qadir (6/173)
Is Witr part of Qiyam al-layl?
It should be understood that Witr does not come under the heading of Qiyam al-layl, so it cannot be understood from the fact that it is valid to pray Witr with one rak`ah, that the minimum for Qiyam is one rak`ah too.
It says in Kashshaf al-Qina‘ (5/23): Is Witr part of Qiyam al-layl or something else? There are two possibilities, the most likely which is the second, which is that Witr is not part of Qiyam al-layl, because of the hadith quoted by Ibn ‘Aqil: “Witr, tahajjud and two rak`ahs of Fajr.” Shaykh Taqiy ad-Din said: Here our companions differentiated between Witr and Qiyam al-layl.”
Thirdly:
Strive hard to offer Qiyam al-layl 
It is no secret Qiyam al-layl requires effort and preparation , especially in Ramadan, and the more prayers a person does, the greater his reward will be, because for every prostration Allah will raise the person one degree in status. 
Muslim (488) narrated that Ma‘dan ibn Abi Talhah al-Ya‘muri said: I met Thawban, the freed slave of the Messenger of Allah (blessings and peace of Allah be upon him), and said: Tell me of a deed that I may do, by which Allah may admit me to Paradise – or he said: Tell me of the most beloved of deeds to Allah. He remained silent. I asked him (again) and he remained silent. I asked him a third time and he said: I asked the Messenger of Allah (blessings and peace of Allah be upon him) about that and he said: “You should prostrate to Allah a great deal, for you will not perform one prostration to Allah but Allah will raise you one degree in status thereby and erase one sin for you.” Ma’dan said: Then I met Abu’l-Darda and asked him (the same question), and he said the same as Thawban had said to me.
It is most unfortunate for a person to limit himself to doing two rak`ahs, except for a serious reason that may happen on some nights, such as if he is too tired, or too busy doing work that prevents him from praying and he is unable to delay the work until another time when he is free. But when one is able to do it and is free, not praying more may be indicative of weak resolve, or it may be a sign of carelessness and weak faith.
So strive hard and persist, make your intention sincerely for the sake of Allah Alone, and strive against your self which is inclined to be lazy; push yourself to do acts of worship until it becomes easy for you to do them.
We ask Allah to guide and help us and you.
And Allah knows best.