The cut offs for jee main will be anounced shorly till then we can't be absolutely sure about the cut off for any category because cut offs may depend upon many factors like number of applicants fill the form, number of applicants finally appeared in the exam, number of questions asked in the exam, difficulty level of the exam, performance of the candidate, previous year's cut offs trends but if we gave a look upon previous years cut for SC it was around 28 or something so the opening and closing rank will be according to it only. But if you have calculated or have an estimation of your marks you may your rank on career 360 rank predictor then can check the last years opening and closing rank for different courses in different NITs. 

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

https://engineering.careers360.com/jee-main-rank-predictor

Good Luck!!