Four rak'ahs, but sometimes more as he pleased.
Sahih Muslim Book 6, Hadith 93
← Previous Hadith I have never seen the Messenger of Allah (ﷺ) observing the supererogatory prayer of the forenoon, but I observed it. And if the Messenger of Allah (ﷺ) abandoned any act which he in fact loved to do, it was out of fear that if the people practised it constantly, it might become obligatory for them.

Sahih Muslim Book 6, Hadith 92 Next Hadith → " As Allah pleased."

Sahih Muslim Book 6, Hadith 94