I have got 74.6% in 12 board exam,it will be sufficient for jeemains or I have to give improvement exam in 2021?

Hello

There's no percentage barrier to be eligible for JEE mains

So , with 74.6 % you are definitely eligible for JEE Mains exam ination , so  you don't need to appear for improvement examination to be eligible for JEE Mains

But yes,

To be eligible for NITs, IIITs, GFTIs , jee advanced( for iits) one needs at least  75% marks in aggregate in class 12 if belong to general or obc category or at least  65% marks if belongs to SC/ST or  must be in the top 20 percentile in  class 12 examination.

So , if  you are from general category and want to get in a seat in NITs, IITs etc then only you need to appear for improvement examination to get at least 75%

For detailed information go through

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

Thankyou