How many people were sentenced to prison in 2011 in the United States? This statistic is hard to come by online. The only measured incarceration rate I've come across is the amount of people incarcerated per 100,000 people.
The DoJ published this paper in December 2011 and released prison statistics for 2010. According to the article the number of people in prison at year-end'10 was 7.1 million of which 2,226,800 people were sentenced in 2010 itself [table 1, in that paper].
This wikipedia article also has a lot of detail on this subject. I think you can extrapolate the figures in the DoJ article with the 2011 rates mentioned in the wikipedia article and compute the number of people in Jail in 2011.