Hello Aspirant,
To my knowledge the cut offs are based on the ranks, so you will get your college on the basis of the rank you get in the exam. And since this time there will 2 phases of the exam therefore the best of two will be considered. For your better understanding I am attaching a link below which will give you the previous year cut offs for the top NITs. You can visit the link and atleast get some idea what are the cut off ranks for the same.
https://engineering.careers360.com/articles/jee-main-cutoff-for-top-nits

Hope this answers your question well. Good Luck.