Deleting rows from Oracle table user_scheduler_job_run_details
You can specify it in days, not number of rows.

DBMS_SCHEDULER.PURGE_LOG


  By default, the Scheduler automatically purges all rows in the job log
  and window log that are older than 30 days. The PURGE_LOG procedure is
  used to purge additional rows from the job and window log.
  
  log_history
  
  This specifies how much history (in days) to keep. The valid range is
  0 - 1000000. If set to 0, no history is kept.


DBMS_SCHEDULER.PURGE_LOG (
   log_history             IN PLS_INTEGER  DEFAULT 0,
   which_log               IN VARCHAR2     DEFAULT 'JOB_AND_WINDOW_LOG',
   job_name                IN VARCHAR2     DEFAULT NULL);