How many rakʿahs is the Duha prayer, and why pray it?

Short answer

The Prophet ﷺ said that every joint of the body owes a charity each morning, and that two rakʿahs prayed in the forenoon (Duha) suffice for all of it. Set a mid-morning reminder labelled "Duha: 2 rakʿahs" and pray them when it rings.