5 Democrats, 8 Republicans, according to Wikipedia (and virtually all of them are running for something else). In contrast, 40 Congressmen retired during 2012’s election cycle, 37 during 2010’s, 33 during 2008’s, and 28 during 2006’s*. So either we’ve got a bunch of retirements coming up – always a possibility, although primary deadlines aren’t that far away – or maybe we’re finally running out of low-hanging Congressional fruit.
I don’t have an answer on this, actually; it just seemed of interest.
*Mostly more Republicans than Democrats in the last four cycles (more Democrats than Republicans in 2012).