For some recent history:
In 2000, five Senators retired (four Democrats, and one Republican). In 2002, four Senators retired, all Republican, including Strom Thurmond, the only Senator to have reached 100 years of age while still in office. In 2004, eight Senators retired (five Democrats, and three Republicans) and all but one yielded his seat to the opposing party. And in 2006, four Senators retired (two Democrats, one Republican, and the independent Jim Jeffords).
Six is the highest number of Republican retirements in a half-century, but five retired in 1978 and 1996.