Hello Gaurav,
I am writing this answer assuming that you are talking about the improvement exams conducted by NIOS for students who have passed class 12th examinations the previous year. If so, then yes, you can apply for improvement in any number of subjects and the marks obtained in other subjects will not change. Although do keep in mind that some universities and examinations require you to appear in improvement examinations for all the subjects and not just some subjects.
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