The result for JEE main 2020 January session is already declared and you can check the result by visiting the link given below

https://www.careers360.com/question-jee-main-2020-result-declared-check-yours-now

from the percentile scores whatever you got in the result you can calculate the rank using the following formula

(100- percentile score )*869010/100

the number of candidates appeared for JEE main January session is 869010

If the number of total candidates appearing for JEE main April session get increased to 12 lakh then the formula would be

(100-percentile score)*1200000/100

these ranks are approximate and the final rank will be come only after the result of JEE main April session