In the 2012 elections, 33 Senate seats were contested. Ten of these were open seats due to incumbent retirements. That leaves 23 seats that an incumbent contested.
One incumbent was defeated in the primaries: Richard Lugar (R) of Indiana.
One incumbent lost the general election: Scott Brown (R) of Massachusetts.
All other incumbents were reelected.
That makes 21 of 23 incumbents reelected to the Senate in the 2012 election, (or 2 incumbent Senate losses, depending on your perspective), making for a 2012 Senate reelection rate of 91%. This is slightly higher than historical standards (where the average is right around 85% for the Senate), but not out of the ordinary.
As of the time of this writing, there are 12 House seats yet to be decided. I’ll post House reelection data as soon as the information becomes available.