Hello,
It is not mandatory to appear for the all the subjects for the improvement exam.
You need to have minimum 75% in aggregate if you are from general category or 65% in case of ST/SC/OBC or you must be in the top 20 percentile of your respective board.
Here are the rules for improvement exam

• If you appear for a single subject for improvement exam, then the cutoff for top 20 percentile for the previous year will be used
• if you appear for all the subjects for improvement exam, then the best of the two performance will be used.
• In case of single subject, the aggregate would be calculated by taking in account the best score for the particular subject

For more details about JEE mains eligibility criteria please visit this link https://www.google.com/url?sa=t&source=web&rct=j&url=https://engineering.careers360.com/articles/jee-main-eligibility-criteria/amp&ved=2ahUKEwi87bbv_dHqAhV683MBHaxbDVcQFjALegQIAxAB&usg=AOvVaw0Ddf__yVIUe0laa-JjEit-&ampcf=1
hope it helps

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