Hello Rishikesh. The link given below will guide you to calculate your percentile and will help you know if you are in the top 20 percentile. Ignore the iit part just refer this link to know your percentile ranking.

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