Hello Diganta,

There is no PREP rank in IIT  or in JEE advance 

There  it is done as the improvement of SC/ST candidates who are unable to qualify in competative exam and there are generally 30-40 seats available for the PREP course and the duration is for 1 year and you will be trained in IIT only and it just depends when the reserved seats remain vacant and can't be filled during the councelling session 

There you have to score min of 2.5% of marks in each subject and 5% of aggregrate marks to get to PREP session 

Hope this will help you and can ask for further clarification