Shama-il Tirmidhi Chapter 040, Hadith Number 001 (272).

Mu'aadhah Radiyallahu 'Anha says: "I asked 'Aayeshah, 'Did Rasulullah Sallallahu'Alayhi Wasallam perform the salaatut duha (chaast prayers) ?" She replied: "Yes, he performed four rak'ahs (the least), and added to it the amount Allah Ta'aala willed".

Commentary
Salaatut duha is a nafl devotion, therefore at least two rak'ahs could be performed. More could be added to it, if the heart wills, which has no end. It has been ascertained that Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam performed up to twelve rakahs. Some are of the opinion, that it has been ascertained that Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam only performed eight rak'ahs. But one has certainly been encouraged in a narration to perform up to twelve rak'ahs.