Sunnah Prayers: 10 or 12?
Question

In his Sahih, Al-Bukhari narrated that ‘Abdullah ibn ‘Umar (may Allah be pleased with him) said: I prayed with the Messenger of Allah (blessings and peace of Allah be upon him) two Rak’ahs before Dhuhr, two Rak’ahs after Dhuhr, two Rak’ahs after Jumu‘ah, two Rak’ahs after Maghrib, and two Rak’ahs after `Isha’. Are the Sunnah prayers mentioned in this Hadith the regular Sunnah prayers? If that is the case, can they be offered in congregation? Can they be included with another prayer with one intention?
Praise be to Allah.Number of the regular Sunnah prayers
The regular Sunnah prayers mentioned in the Hadith narrated from Ibn `Umar (may Allah be pleased with him) are ten Rak’ahs. According to the correct view, the regular Sunnah prayers are twelve Rak’ahs, because of the Hadith of `Aishah and Umm Habibah (may Allah be pleased with them), which mentions four Rak’ahs before Dhuhr .
Shaykh Ibn Baz (may Allah have mercy on him) said: 
“The regular Sunnah prayers are twelve Rak’ahs, although some of the scholars were of the view that they are ten. However, there are proven reports from the Prophet (blessings and peace of Allah be upon him) which indicate that they are twelve Rak’ahs, and that the regular Sunnah prayer before Dhuhr  is four Rak’ahs.
`Aishah (may Allah be pleased with her) said: The Prophet (blessings and peace of Allah be upon him) never omitted to pray four Rak’ahs before Dhuhr  . 
As for Ibn ‘Umar (may Allah be pleased with him), it is soundly narrated from him that they [the regular Sunnah prayers] are ten Rak’ahs, and that the regular Sunnah prayer before Dhuhr  is two Rak’ahs, but `Aishah and Umm Habibah (may Allah be pleased with them) learned from the Prophet (blessings and peace of Allah be upon him) that it is four Rak’ahs, and the basic principle is that the one who learned [something from the Prophet (blessings and peace of Allah be upon him)] is the one who is correct, not the one who did not learn that from him.
Thus it is established that the regular Sunnah prayers are twelve Rak’ahs: four before Dhuhr  and two after it, two after Maghrib, two after `Isha’ and two before Fajr prayer." (Majmu‘ Al-Fatawa 11/281)
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said: 
“The author regarded the number of regular Sunnah prayers as ten, basing that on the Hadith of `Abdullah ibn `Umar (may Allah be pleased with him), who said: I learned from the Messenger of Allah (blessings and peace of Allah be upon him) ten Rak’ahs – and he listed them. This is one of the two views concerning this matter.
The second view concerning this matter is that the regular Sunnah prayers are twelve Rak’ahs, based on the Hadith which was narrated in Sahih Al-Bukhari from `Aishah (may Allah be pleased with her), who said: The Prophet (blessings and peace of Allah be upon him) never omitted to pray four Rak’ahs before Dhuhr . 
It was also narrated soundly that he (blessings and peace of Allah be upon him) said: “Whoever prays twelve Rak’ahs apart from the obligatory prayers, Allah will build for him in return for them a house in Paradise.” And he mentioned among them “four Rak’ahs before Dhuhr ” and the others as noted above.
Based on that, the correct view is that the regular Sunnah prayers are twelve Rak’ahs: two Rak’ahs before Fajr; four Rak’ahs before Dhuhr , with two Salams, and two Rak’ahs afterwards; two Rak’ahs after Maghrib; and two Rak’ahs after `Isha’." (Ash-Sharh Al-Mumti‘  4/68) 
Ash-Shawkani (may Allah have mercy on him) said: 
“Ad-Dawudi said: In the Hadith of Ibn `Umar, it says that there are two Rak’ahs before Dhuhr  prayer, and in the Hadith of `Aishah it says that there are four. This may be understood as indicating that each of them described what he or she saw. It may also be that Ibn ‘Umar forgot two of the four Rak’ahs.
Ibn Hajar (may Allah have mercy on him) said: This understanding is far-fetched. What is more likely to be the case is that both scenarios are correct, meaning that the Prophet (blessings and peace of Allah be upon him) sometimes prayed two Rak’ahs and sometimes prayed four.
And it was said that it is to be understood as meaning that in the mosque he would limit it to two Rak’ahs and in his house he would pray four.
It may be that when he was in his house he would pray two Rak’ahs, then he would go out to the mosque and pray two more, and Ibn ‘Umar was only aware what happened in the mosque, not what happened in his house, whereas `Aishah was aware both.
The former view is supported by the Hadith which Ahmad and Abu Dawud narrated from `Aishah, that the Prophet (blessings and peace of Allah be upon him) used to pray four Rak’ahs in his house before Dhuhr , then he would go out.
Abu Ja‘far At-Tabari (may Allah have mercy on him) said: Four Rak’ahs is what he usually did, and on rare occasions he prayed two Rak’ahs [before Dhuhr ]." (Nayl Al-Awtar  3/21). 
Shaykh ‘Abd Al-Muhsin Al-‘Abbad (may Allah preserve him) said: 
“The Hadith of Umm Habibah and the Hadith of `Aishah agree in terms of the number, and that the regular Sunnah prayer before Dhuhr  is four Rak’ahs. This is in contrast to the Hadith of Ibn ‘Umar, in which it says that it is two Rak’ahs before Dhuhr . Undoubtedly it is more appropriate to do that which is more complete and better, namely four Rak’ahs, and that whoever prays two Rak’ahs has done well and there is nothing wrong with that." (Sharh Sunan Abu Dawud) 
Are the 2 Rak’ahs after Jumu`ah among the regular Sunnah prayers?
These two Rak’ahs , which are mentioned in the Hadith of Ibn ‘Umar (may Allah be pleased with him) as being offered after Jumu‘ah prayer, are not among the regular Sunnah prayers that are to be repeated every day and night. Rather this is a separate prayer which is not included in the number mentioned in the Hadith of Ibn ‘Umar that is quoted above.
As-San`ani (may Allah have mercy on him) said: The words “ten Rak’ahs” refer to what is repeated every day." (Subul As-Salam  1/316)  
Shaykh ‘Abd Ar-Rahman As-Suhaym (may Allah preserve him) said: “The Hadith of Ibn `Umar refers to the [Sunnah] prayer after Jumu`ah; this is not one of the regular Sunnah prayers; rather it is a separate prayer." (Sharh Al-‘Umdah  1/209)  
Can one pray Sunnah in congregation?
The basic principle regarding supererogatory prayers and regular Sunnah prayers is that they should be offered individually , except in cases in which the Sunnah is to offer them in congregation, such as Tarawih prayers , the eclipse prayer , and so on.
Nonetheless, if these supererogatory prayers are offered in congregation sometimes, or there is a reason for doing that, then there is nothing wrong with it, but that should not be taken as a regular practice or as something that people get together and do regularly.
Can one combine two Sunnah Rak’ahs of `Isha’ and Qiyam Al-layl with one intention? 
The Hadith of `Aishah (may Allah be pleased with her) should not be understood as meaning that the two Sunnah Rak’ahs of `Isha’ may be put together with the night prayers with one intention , because it may be that the Prophet (blessings and peace of Allah be upon him) prayed `Isha’, then prayed the regular Sunnah prayer after it, but the narrator did not mention that, just as he did not mention Witr. It may also be that the narrator specifically meant by that the night prayer.
And Allah knows best.