Hello Sunita Pandey

The JEE Main eligiblity criteria is that the candidate should have passed in 2018 or 2019 or is giving his 12th in 2020 are eligible.

However for admission in NITs, IIITs, GFTIs the candidate must have atleast 75% marks or in the top 20 percentile in his respective board. ie, whether you have to be in the top 20 percentile depends upon the board in which you are giving your exam.

Kindly check the mentioned below links for more details.

https://engineering.careers360.com/articles/jee-main-eligibility-criteria/amp

https://engineering.careers360.com/articles/top-20-percentile-cutoff-of-class-12-boards-for-iit-admission

Hope it helps you.