See if you want to take admission in any of the IITs for B.Tech course then you have to appear in JEE-Advance and secure good enough rank to get your preferred IIT and apart from this it is also mandatory that your minimum total aggregate of marks in class 12th should be 75% for General/Unreserved category and 65% if you belongs to any of the reserved category. Suppose if you are unable to secure the above mentioned percentage then you are not eligible for admission to any of the NITs/IITs/IIITs and so on even after securing good rank in JEE-Main or JEE-Advance 2021.Also note that the 75% criteria is only to determine your eligibility and your admission will not be done on the basis of class 12th percentage so even if you get 90% or 99% in class 12th boards but unable to qualify JEE-Advance then you are not eligible for admission to IITs.
For good IITs such as IIT Bombay, IIT Kharagpur, IIT Kanpur, you should secure your rank in JEE-Advance 2021 under at least 1000.
For complete details about JEE-Advance kindly check out the link given below :-
I hope this information helps you.