Hi dear,

You have to score 75 percent in board or should be in top 20 percentile.This is necessary, when you go for reporting in case you are not in top 20 percentile or 75 percent in board.You cannot claim a seat in IITs or NITs.Many board have cutoff less than 60 so they can't take admission in IIT MADRAS.

HOPE it helps

Hii

To get admission to IITs you need to fulfill the eligibility criterias which are as follows - 

• You must qualify class 12th examination with subjects physics, chemistry, maths and english.
• The candidate must have secured atleast 75% aggregate in the examination or must be among the top 20 percentiles in their respective boards.
• The candidate must be of atleast 17 years of age.
• The candidate must qualify JEE Mains and secure a position among top 224000 candidates.
• The candidate must appear and qualify JEE Advance.
  

This is a short description of the eligibility criteria. A simple answer to your question is that you require 75% aggregate in class 12th. 
For a detailed information about the eligibility, you can use the following link - https://engineering.careers360.com/articles/jee-advanced-eligibility-criteria

Hope this helps
Thanks!!