Fiqh-us-Sunnah Fiqh 2.31a
The time for ‘duha’ begins when the sun is about a spear’s length above the horizon and it continues until the sun reaches its meridian. It is preferred to delay it until the sun has risen high and the day has become hot.
Zaid ibn Arqam (r.a.)relates: “The Messenger of Allah (s.a.w.s) went to the people of Quba’, and they were performing the ‘Duha’ prayer, and he (s.a.w.s) said: ‘The prayer of ‘Duha’ should be observed when the young weaned camels feel the heat of the sun.'”
Related by Ahmad, Muslim, and at-Tirmidhi .

Fiqh-us-Sunnah Fiqh 2.31b
The minimum number of rak’ah to be prayed in the supererogatory ‘Duha’ (forenoon) prayer is two. The most the Prophet (s.a.w.s) performed was eight rak’at, whereas, the most he (s.a.w.s) mentioned was twelve rak’at. Some eminent scholars, such as Abu Ja’far At-Tabari, Al-Mulaimi, and Ar-Ruwyani, who subscribes to the Shafi’ school of thought, say there is no maximum limit to the number of rak’at that one may perform for the voluntary ‘Duha’ prayers.
Al-‘lraqi says, in the commentary on Sunan At-Tirmidhi: “None of the companions of the Prophet (s.a.w.s) or followers are known to have restricted it to twelve rak’at.” As-Syuti agrees with it.

Sa’id ibn Mansur (r.a.) records that al-Hassan (r.a.) was asked: “Did the companions of the Prophet (s.a.w.s) perform the ‘Duha’ prayer?” He answered: “Yes . . . some of them would pray two rak’at, and some of them would pray four rak’at, and some of them would continue until half the [early] day [had passed].”

Umm Hani (r.a.) narrates that the Prophet sallallahu ‘alehi wa sallam prayed eight rak’at of ‘Duha’ and made the taslim after every two rak’at.
Related by Abu Dawud.

Aishah (r.a.) reports: “The Prophet (s.a.w.s) would pray four rak’at for ‘Duha’ and would add to it whatever Allah willed.”
Related by Ahmad, Muslim, and ibn Majah.

Sahih Al-Bukhari Hadith 2.274 Narrated by Abu Huraira (r.a.)
My friend [the Prophet (s.a.w.s)] advised me to do three things and I shall not leave them till I die, these are: To fast three days every month, to offer the voluntary Duha prayer, and to offer Witr before sleeping.

In light of the above guidance of the Messenger of Allah (s.a.w.s), it would be considered a Sunnah to offer the voluntary ‘Duha’ (forenoon) prayers between the time the sun has completely risen until it reaches its zenith or meridian at mid-day. One may offer a minimum of two rakahs, and a maximum of as much as one wishes.