Hello student The formula to calculate the rank with the help of percentile given by NTA is as mentioned below -

100-(your total percentile) *Total num of Aspirants /100.

In Cas if you're concerned about Jee main january examination the total number of Aspirants were 869010.

You can also use careers360 college predictor to predict college. - https://engineering.careers360.com/articles/jee-main-college-predictor/amp

Feel free to comment if you've any doubt

Good luck