st students can get free seats upto which percentile

Hello, please mention the college in which you want admission. Based on colleges the percentiles changes for free seats. Many factors are responsible for this, like college cut-off, competition, number of SC candidates joining this year, etc.
First you should qualify in JEE Mains if not without qualify no free seat is given. To get free seat not just qualifying is enough, you should get good rank among the people in your category to get free seat.

This year expected cut-off:

Common Rank List (CRL) - 90- 95
GEN-EWS - 70- 75
Other Backward Classes (OBC-NCL) - 70- 75
Scheduled Caste (SC) - 50- 55
Scheduled Tribe (ST) - 40- 45
PwD - 1-2

Previous year's cut-off:

Common Rank List (CRL) - 90.3765335
GEN-EWS - 70.2435518
Other Backward Classes (OBC-NCL) - 72.8887969

To know previous year's cut-offs click below links.

Cutoff-

https://engineering.careers360.com/articles/jee-main-cutoff

Official cutoffs for NITs, IIITs, IITs, CFTIs-

https://josaa.nic.in/Result/Result/currentorcr.aspx

To predict colleges go through below link.

https://engineering.careers360.com/jee-main-college-predictor?icn=QnA&ici=qna_answer