The top 20 percentile varies every year and is different according to the category to which you belong to. In the top 20 percentile the percentage marks are provided for the top 20 percentile students, although you are eligible if you have 75%+ marks in aggregate. You can check the top 20 percentile marks here:-

https://josaa.nic.in/webinfocms/Handler/FileHandler.ashx

Hi Divyam! 

Well to be honest no one can know except your own board and JoSAA it self.

Every board have to provide top 20 percentiles list to JoSAA. If that board could not provide T20 percentiles list then you have to go to your board office and get a certificate written that you are officially in top 20 percentiles of your respective board.

Is that complicated then there's easy way you have to just obtain 75% (for general ) or 65%(for St/sc) in cbse board. 

Please upvote if this was helpful to you. 

Thank you and good luck.