From Wikipedia: Elections to the United States Senate are to be held on November 6, 2012, with 33 of the 100 seats in the Senate being contested in regular elections whose winners will serve six-year terms from January 3, 2013 until January 3, 2019. Additionally, special elections may be held to fill vacancies that occur during the 112th United States Congress. Currently, Democrats are expected to have 23 seats up for election, including 2 independents who caucus with the Democrats, while Republicans are expected to have only 10 seats up for election.
So 33% of those up for relecection., 7 out of 21 are "retiring". NICE!