Sayyidna Anas, Radi-Allahu anhu, reports that
Rasulullah (Sallallaahu Alayhi Wasallam) said, 'Whoever performs 12 Rakaats of Salat al-Dhuhaa, Allah Ta' ala will build for him a castle of gold in Jannah.' (Mishkaat pg.116)
It is reported that Sayyidna Aaisha (Radhiallaahu Anha) used to perform 8 Rakaats of Salat al-Dhuhaa. She said that even if her parents arose from the grave, she would not leave Salat al-Dhuhaa to go and meet them. (Ibid). Rasulullah is reported to have said, 'Whoever offers 2 Rakaats of Salat al-Dhuhaa, all his sins will be forgiven even though they are as much as the foam of the sea.' (Ibid)
'Salat al-Dhuhaa consists of 2 - 12 Rakaats and it is preferable to perform 8 Rakaats.' (Raddul Mukhtaar vol.1 pg.505)
The Mustahabb (preferred) time of performing Salat al-Dhuhaa is after 1/4 of the day has passed. This is deduced from a Hadith of Muslim. (Halabi; Sharah Kabeer pg.390). From many Ahaadith, we understand that Salat al-Dhuhaa should be performed after the sun has risen quiet high. Because the Fuqahaa and Muhadditheen are of the opinion that the commencement of the time of Salat al-Dhuhaa is the same as that of Ishraaq (i.e. immediately after sunrise), the Ulama have stated that 'if due to lack of time, one performs the Salat of Ishraaq and Dhuhaa together at one time, that too will be correct.' (Our Namaas pg.44)
And Allah Ta'ala Knows Best