In order to get admission in IITs, NITs, IIITs you should have minimum 75% in class 12th & 65 % as a reserved category or you should be in top 20% percentile in your state in your class XII exam conducted by the respective board. You can check the further eligibility with the given below link.
Yes top20 percentile of respective boaed or 75%aggregate marks in 12th is foundation of eligible criteria for NIT and IIITs via Jee Main.
All NITs shall expected to fulfill above criteria..
Top 20 percentile cutoff is different flr all baords and category .