Hello,
You must have an aggregate of 75 percentage in your five subjects in your CBSE Class 12th boards or you should be in top 20 percentile of your respective boards in order to get admission in NITs, IIITs and IITs. If you want to give CBSE improvement examination to be held next year along with present students of Class 12th, you can give in desired number of subjects if you have passed them and you are not satisfied with the marks. You can give improvement examination in as many subjects you want.  It is up to you. You are eligible for admission in NITs, IIITs and IITs based upon your JEE rank after giving improvement examination.
I hope this helps. For any further queries, ask me in the comments.