What is the cutoff marks to reserve a sit in iit Bombay in CS???

Cut off marks can be different each year and won't be a precise way to generalise it,but you need to be in top 59 in JEE Advanced final Open Ranking to grab it.

The cutoff is the minimum rank that a student will have to secure to be eligible for admission  in IIT's..Cutoff vary according to the course of the participant and their category..Jee advance cutoff for IIT Bombay will be prepared by official authorities by taking a variety of factors into consideration..The cutoff of jee advance of IIT Bombay consists of opening and closing rank needed for admission.Only those applicants who meet the cutoff will be termed as eligible for admission in this institute.Ineligible candidates will be disqualified ..Cutoff of the exam is prepared  on the basis of no. of applicants , previous year cutoff trend, performance of the applicants and many more .. The cutoffs will  not remain same every year it has some difference but not so high , a slight difference in marks.
In the previous year (2017) , the cut off for the Computer Science Engineering in IIT Bombay is as follows ...
Category      opening rank              closing rank

General                 1                                      62
OBC NCL               9                                      45
SC                            1                                      22
ST                             4                                      15