This is a matter of serious historical dispute, and it's difficult to arrive at numbers with any degree of certainty.

Frank Dikotter wrote a book called Mao's Great Famine several years ago and stated that while orthodox estimates put the number at about 30 million people, his research (conducted during a period when the Chinese government briefly threw open the state archives around the turn of the century) suggests that more like 45 million people died. Dikotter also suggested that the famine lasted for almost five full years rather than the often stated three, and that millions were deliberately starved deliberately by Chinese authorities.