Dear aspirant

I am afraid to confirm you that " preparatory rank" is not really a rank . Every IIT has separate quota seats for SC , ST , OBC etc. These seats ho vacant in every year . So the vacant seats are allotted to students of that respective category for a one year course . If they pass then they are eligible for undergraduate courses at IITs and they get direct admission in those . However students  doing preparatory courses can sit for JEE ADVANCED next year also.

You can predict your chances in IITs by actual JEE ADVANCED rank here

https://engineering.careers360.com/articles/jee-advanced-rank-predictor

Best wishes.