You can calculate your rank by the following formula

(100-Percentile score)*869010/100, Therefore your rank is 33978

If the number of student get increased to 1200000 then the rank would be

(100-Percentile rank)*1200000/100 ,which is 46920

Admission to IIIT depends upon category rank as well as the branch you are interested in , so based on specific condition the closing rank is different you can check the closing rank for your interested course by visiting the link given below.

Opening Closing rank (https://josaa.nic.in/webinfocms/Handler/FileHandler.ashx?i=File&ii=177&iii=Y)

Meanwhile you can use JEE MAIN college predictor

JEE MAIN college predictor