JEE Advanced

Yes, you can give improvement exam for any subject of 12th class for meeting the JEE Main eligibility criteria of 75% aggregate marks. The CBSE improvement exams form will be available in the month of October every year.

The IIT-B cutoff for B.Tech courses are varying according to specializations. The cutoff ranks of CSE goes very high. The last year cutoff ranks of some specializations are given below for an idea.
1. CSE (Closing rank 21)
2. Civil (Closing rank 661)
3. Electrical (Closing rank 100)
4. Aerospace (Closing rank 564)
5. Chemical (Closing rank 604)
6. Mechanical (Closing rank 250)
So from above you have to score at least 220 marks in JEE Advanced for getting admission here in Civil engineering branch.

I am sending you an authentic link from where you get all information about the cutoff for IIT CHENNAI. The cutoff are on the basis of different types of parameters. You check there. link-https://www.shiksha.com/college/iit-madras-indian-institute-of-technology-adyar-chennai-3031/cutoff.

Please use this college predictor tool by Shiksha. It will help you to predict the college according to your rank or percentile:
I wish you a bright future. https://www.shiksha.com/college-predictor.