In your .PLR file you have two entries like these below:
Number Track Replays="2" // how many replays to store for each track (using default naming convention only!)
Number Race Results="2" // how many results files to store (using default naming convention only!)
I got mine on 2 because I don't want to keep several replays (can't recall what is the default). So everytime that I race a third time in a same track, the oldest replay get replaced, so I'll have only the two more recent in the fridge.
Keep in mind that this number is valid for "default names", i.e., if you rename a replay, it will not be considered for removal anymore.