Hello,
It is not necessary to give improvement exam in all the subjects. You can appear for improvement in one or more subject in which you have scored less marks for getting the requisite 75% in Class 12th.
However, if you wish to get into the top 20 percentile then you will have to appear for all the five subjects.

You need to also understand the following points about the improvement exam-
If you appear for improvement in one or more (but not all) subjects then the 12th percentile marks of your original passing year will be considered.

If you appear for improvement in all subjects, the percentile marks of the present session will be taken into consideration.

Good Luck!