It is recommended to pray Duha, the least of which is two rak`ah, the best of which is eight, and the average of which is four or six, and encouragement to do so regularly Sahih Muslim 721 a
Translation
Abu Huraira reported. My friend (the Holy Prophet, may peace be upon him) has instructed me to do three things
three fasts during every month, two rak'ahs of the forenoon prayer, and observing Witr prayer before going to bed.