Greetings!

To start with, it is impossible to guess how many marks does someone need in advanced to get into IIT, it is because of the fact that the marks of JEE advanced are NOT fixed, no one on earth can tell for how many marks the exam will be conducted until it actually is being conducted! With that thing clear let's move to some estimates that we can make.

As a ST student how much percentile you need can be guessed, form previous year cutoffs, to get into any IIT in any meaningful branch your category rank must be under 1000, now total number of ST candidates appearing for JEE changes form year to year, but hovers around 21K, so that is your mark, first you will have to beat them in race to qualify for advanced and then in advanced

You can use the following predictor to check which colleges you can get at which ranks

https://engineering.careers360.com/jee-advanced-college-predictor?utm_source=qna&utm_medium=jee_adv_cp