As we go through the latest cut-offs of 2020 then it shows that the closing rank for OBC category was 2185 for Metallurgical Engineering and Materials Science branch in IIT Bombay.
See it is quite difficult to tell you about exact marks to score because for JEE-Advance no maximum marks are fixed but yes with regards to JEE-Advance 2021 if he secures his AIR under OBC category in JEE-Advance under top 2000-2100 then there are good chances for him to get into some lower branches in IIT Bombay but if he wants top branches like CSE, ECE, or any other then he should maintain his rank under 200 for IIT Bombay.
You can check out the latest cut-offs from the link given below :-
Note that the above prediction is made on the basis of latest cut-offs available on official website of JOSAA and the cut-offs are bound to change every year depending upon various factors such as the number of candidates appearing in an examination, the number of candidates qualifying an examination, difficulty level of the paper, and so on.
To have a list of all those colleges which you can get based upon your JEE-Main score, category, and state you can use JEE-Main College Predictor :-
As you are preparing for JEE-Main or JEE-Advance 2021 the I would recommend you to check the resources made by our experts to help aspirants like you in the best and smart preparation of JEE-Main or JEE-Advance 2021 :-
I hope this information helps you.