Soviet authorities, who have the most to lose from a high estimate, believe that between 2.5 and 3.5 million people died during the Holodomor. Outside historians put the total as high as 14 million. Very few put the total as low as the number that perished in, say, the Holocaust.
With Chairman Mao's excursions into Central Economic Planning being the only exception, this famine is the worst thing that any government has ever done to its own people.
Apologists from the left are embarrased by it, which is why you didn't learn about this in your High School history text. It was the ultimate big government intervention into the free market. And it was planned, implemented, and later justified as being for a good cause (eliminating a troublesome social class, ending individual ownership, taking over the means of production, etc.)
Chances are, you've never heard of it.