Hello, aspirant firstly your preparatory rank is not a rank in JEE Advanced. The seats which are reserved for SC,ST ,PWD candidates left vacant are allowed to you for a preparatory course.
Even then your chances og getting an IIT is very less because around 8000 sc students qualified JEE advanced this year . I suggest you to go for some private colleges.
To check no of sc,st students qualified go through this link
https://www.google.com/url?sa=t&source=web&rct=j&url=https://engineering.careers360.com/articles/jee-advanced-2019-toppers/amp&ved=2ahUKEwjC7pGcsu_iAhUHMo8KHQK0ASIQFjANegQICRAK&usg=AOvVaw37HI8SDk458NUGpEPZW7Fi&ampcf=1

Hope this helps you.
Good luck.

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