DUHA PRAYER & MINIMUM RAKAHS IN DUHA PRAYER
Abu Dharr reported Allah's Apostle (ﷺ) as saying:
In the morning charity is due from every bone in the body of every one of you. Every utterance of Allah's glorification is an act of charity. Every utterance of praise of Him is an act of charity, every utterance of profession of His Oneness is an act of charity, every utterance of profession of His Greatness is an act of charity, enjoining good is an act of charity, forbidding what is distreputable is an act of charity, and two rak'ahs which one prays in the forenoon will suffice.
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدِ بْنِ أَسْمَاءَ الضُّبَعِيُّ، حَدَّثَنَا مَهْدِيٌّ، - وَهُوَ ابْنُ مَيْمُونٍ - حَدَّثَنَا وَاصِلٌ، مَوْلَى أَبِي عُيَيْنَةَ عَنْ يَحْيَى بْنِ عُقَيْلٍ، عَنْ يَحْيَى بْنِ يَعْمَرَ، عَنْ أَبِي الأَسْوَدِ الدُّؤَلِيِّ، عَنْ أَبِي ذَرٍّ، عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ قَالَ " يُصْبِحُ عَلَى كُلِّ سُلاَمَى مِنْ أَحَدِكُمْ صَدَقَةٌ فَكُلُّ تَسْبِيحَةٍ صَدَقَةٌ وَكُلُّ تَحْمِيدَةٍ صَدَقَةٌ وَكُلُّ تَهْلِيلَةٍ صَدَقَةٌ وَكُلُّ تَكْبِيرَةٍ صَدَقَةٌ وَأَمْرٌ بِالْمَعْرُوفِ صَدَقَةٌ وَنَهْىٌ عَنِ الْمُنْكَرِ صَدَقَةٌ وَيُجْزِئُ مِنْ ذَلِكَ رَكْعَتَانِ يَرْكَعُهُمَا مِنَ الضُّحَى " .
Reference: Sahih Muslim 720
In-book reference: Book 6, Hadith 101
USC-MSA web (English) reference: Book 4, Hadith 1557
http://sunnah.com/muslim/6/101
Abu Huraira reported. My friend (the Holy Prophet, may peace be upon him) has instructed me to do three things:
three fasts during every month, two rak'ahs of the forenoon prayer, and observing Witr prayer before going to bed.
حَدَّثَنَا شَيْبَانُ بْنُ فَرُّوخَ، حَدَّثَنَا عَبْدُ الْوَارِثِ، حَدَّثَنَا أَبُو التَّيَّاحِ، حَدَّثَنِي أَبُو عُثْمَانَ النَّهْدِيُّ، عَنْ أَبِي هُرَيْرَةَ، قَالَ أَوْصَانِي خَلِيلِي صلى الله عليه وسلم بِثَلاَثٍ بِصِيَامِ ثَلاَثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَرَكْعَتَىِ الضُّحَى وَأَنْ أُوتِرَ قَبْلَ أَنْ أَرْقُدَ .
Reference: Sahih Muslim 721 a
In-book reference: Book 6, Hadith 102
USC-MSA web (English) reference: Book 4, Hadith 1558
http://sunnah.com/muslim/6/102
Mua'ada 'Adawiyya reported 'A'isha as saying:
The Messenger of Allah (ﷺ) used to observe four rak'ahs in the forenoon prayer and he sometimes observed more as Allah pleased.
وَحَدَّثَنِي يَحْيَى بْنُ حَبِيبٍ الْحَارِثِيُّ، حَدَّثَنَا خَالِدُ بْنُ الْحَارِثِ، عَنْ سَعِيدٍ، حَدَّثَنَا قَتَادَةُ، أَنَّ مُعَاذَةَ الْعَدَوِيَّةَ، حَدَّثَتْهُمْ عَنْ عَائِشَةَ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّي الضُّحَى أَرْبَعًا وَيَزِيدُ مَا شَاءَ اللَّهُ .
Reference: Sahih Muslim 719 c
In-book reference: Book 6, Hadith 95
USC-MSA web (English) reference: Book 4, Hadith 1551
http://sunnah.com/muslim/6/95
Abd al-Rahman b. Abu Laila reported:
No one has ever narrated to me that he saw the Messenger of Allah (ﷺ) observing the forenoon prayer, except Umm Hani. She, however, narrated that the Messenger of Allah (ﷺ) entered her house on the day of the Conquest of Mecca and prayed eight rak'ahs (adding): I never saw a shorter prayer than it except that he performed the bowing and prostration completely. But (one of the narrators) Ibn Bashshar in his narration made no mention of the word:" Never".
وَحَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَابْنُ، بَشَّارٍ قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ عَمْرِو بْنِ مُرَّةَ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ أَبِي لَيْلَى، قَالَ مَا أَخْبَرَنِي أَحَدٌ، أَنَّهُ رَأَى النَّبِيَّ صلى الله عليه وسلم يُصَلِّي الضُّحَى إِلاَّ أُمُّ هَانِئٍ فَإِنَّهَا حَدَّثَتْ أَنَّ النَّبِيَّ صلى الله عليه وسلم دَخَلَ بَيْتَهَا يَوْمَ فَتْحِ مَكَّةَ فَصَلَّى ثَمَانِيَ رَكَعَاتٍ مَا رَأَيْتُهُ صَلَّى صَلاَةً قَطُّ أَخَفَّ مِنْهَا غَيْرَ أَنَّهُ كَانَ يُتِمُّ الرُّكُوعَ وَالسُّجُودَ . وَلَمْ يَذْكُرِ ابْنُ بَشَّارٍ فِي حَدِيثِهِ قَوْلَهُ قَطُّ .
Reference: Sahih Muslim 336 d
In-book reference: Book 6, Hadith 97
USC-MSA web (English) reference: Book 4, Hadith 1553
http://sunnah.com/muslim/6/97
From all above hadiths, what we can derive is that the minimum Rakah for Duha prayer are two and maximum reported are eight. Majority of the hadiths, however, also indicate the Prophet's (peace be upon him) regular habit was to pray 4 rakah for Duha prayer. And Allah knows the best!