Hello, 

If you are appearing for improvement examination in order to get an aggregate of at least 75 percentage in CBSE Boards or to be in top 20 percentile of your respective boards, you can appear in either one subject or more. There is no such provision for admission in NITs, IITs and IIITs that you need to appear in all the five subjects for your improvement examination. You can appear for as many subjects you want if you have passed all of the subjects. 

I hope it helps. For nay further queries related to this, ask me in the comments.