The 1988-D Kennedy Half Dollar is a scarcer issue than most dates from the same time frame. It has a mintage of about 12 million. Compare this to Kennedy Half Dollars from 1964-2001where these years on average have mintages around 20 million or so, with the exception of some key dates like the 1987-P or 1987-D.
The 1987-D Kennedy Half Dollar is common up to about MS65 condition. Examples in MS66 are a little harder to find but examples still exist in the thousands. In MS67 condition they are much more difficult to find and command a slight premium over MS66 examples. Anything grading higher than MS67 is considered rare.