salat-ul-Duha is an act of Sunnah (reported from the Prophet) and its minimum is two Rak`ahs and it has no maximum limit. However, it is preferable not to exceed eight Rak`ahs and to say Taslim after each pair of Rak`ahs. Furthermore, they should not be combined with one Taslim at the end, for the Prophet (peace be upon him) said:
The (optional) Salah (Prayer) by night and day should consist of pairs of Rak`ahs.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.