20 percentile score means scoring more than 80 percentage students that have given your 12th standard state board examination. It is counted of a total of 5 subjects. In CHSE board there are 6 subjects. So you'll have freedom in ignoring one subject. But you need to consider PCM subjects along with a language subject for sure. These are the basic credentials to find your 20 percentile mark.
Now for CHSE board of Odisha , the 20 percentile cutoff almost always comes around 290-300 out of 500 total marks. So, score above 300 just to be safe and you'll be eligible to join any IITs or NITs of your preference. Every year JOSAA launches the 20 percentile cutoff just before the counseling process of JEE main and Advance starts.
Hope this helps you!
The top 20 percentile means the position at which you are better than 80 percentage students who wrote board exams of odisha.
The cutoff for Top 20 percentile this year for admission to IIT /NIT/IIIT is
General- 293 Marks
OBC -287 marks
SC and st-279 marks
All the very best