there are a lot of factors determining cutoffs like difficulty of the exam, previous trends, number of people attending and how many seats are available.
I cannot predict the cutoff for 2020 but depending on previous years' cutoffs, minimum marks for qualifying for ST category was around 24-30 marks.
for more information go through the following link: https://engineering.careers360.com/articles/jee-main-cutoff
hope this helped
all the best.
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