As per CBSE guidelines, Improvement exams are for students who are not satisfied with their marks and are willing to take the exams again.CBSE Improvement Exam can be taken in one or more subjects in the succeeding year only after passing the subject in Class XII.

NOTE:-

A candidate cannot appear for improvement examination in the subjects which he/she failed to pass.

All the best.

Hi Diwakar

There are no fixed guidelines but I would suggest you to give it once for all and remove your headache.

All the best !