Hi Behera,
- Log on to CMC
- Go to specific Folder where your required report is saved
- Right Mouse Click on it and click "Limits"
- Check "Delete excess instances when there are more than N instances of an object:" Box
- Provide the number of instance you want to keep.
- Click Update Click Refresh Close Limits editor.
Also check:
How to limit number of scheduled instances for a report in..
Note: There are more options available for you to check.
Good luck Bashir Awan