It is an act of Sunnah
to offer two Rak`ahs after `Isha' as part of Al-Sunan Al-Rawatib (supererogatory acts that were stressed and regularly performed by the Prophet) which were related in the Hadith reported by
Um Habibah (may Allah be pleased with her) which is mentioned in the answer to question 3. It is also a Sunnah to offer two Rak`ahs between every Adhan (call to Prayer) and Iqamah (call to start the Prayer) whether for `Isha' or any other Salah because of the Prophet's saying:
There is a prayer between the two calls (Adhan and Iqama).
Muslim. As for the night Salah including Shaf` and Witr, a Muslim may offer the number of Rak`ahs he likes then conclude his Salah by offering one Rak`ah. As for fixing the number of Shaf` to six Rak`ahs, it is invalid and the least permissible number of Rak`ahs is one and the least perfect is to offer three Rak`ahs, while the most perfect is to offer thirteen Rak`ahs in pairs, ending Salah after each two Rak`ahs then offering Witr. Whoever wants to offer more than this number, he is most welcome because the Prophet (peace be upon him) said:
The (optional) Salah during the night should consist of pairs of Rak`ahs, but if one of you fears morning is near, he should pray one Rak`ah, which will make his Salah an odd number for him.
Agreed upon Hadith. The Prophet (peace be upon him) did not specify a certain number of Rak`ahs, and that indicates that the matter is flexible.