To get CSE in IIT Bombay, you need a JEE Advanced rank of 60 if you belong to General Category. This rank keeps getting lower every year. If you are certain of studying CSE at IIT Bombay, then start studying without further hesitation. In case you are still not sure, I have a friend's experience that might help you make the decision. He was interested in Mechanical and was not sure about CSE. With a rank of 70, he went for Mechanical Engineering. 

Now there is a course at IIT Bombay that is compulsory for all freshers. The course comprises the fundamentals of Computer Science. Once he studied that, he became certain that he wanted to pursue CSE instead and requested a branch change. In fact, I know many who changed their branch after becoming aware of their passion and inclinations. So you don’t have to necessarily make a decision right now, and can always avail of the branch change option later on. But make sure to make an early decision. Also if your JEE rank is average then you can always change your branch later on. However, an important thing to keep in mind is that this branch change option is possible only if your first-year CGPA is up to the mark.

To get a seat at IIT Bombay in computer science, one must qualify in JEE Advanced and obtain a high rank. JEE Advanced is the entrance exam for admission to the IITs. The cutoff marks for JEE Advanced vary each year based on various factors such as the difficulty level of the exam, the number of applicants, and the number of available seats. 

In general, a candidate must score well above the minimum cutoff marks to be considered for admission to IIT Bombay. In recent years, those who have secured a rank between 1 and 100 have had a good chance of getting admission to CS in IIT Bombay. However, this is not a fixed criterion and may vary based on the year and the specific circumstances.

The JEE Advanced cutoff 2023 for Aerospace Engineering at IIT Bombay has not been released yet. 

Factors determining IITs Cutoff 2023 for B.Tech Aerospace Engineering are

• Previous years’ cutoff trends
• Difficulty level of JEE Advanced 2023.
• The total number of candidates who appeared for the entrance examination.
• Number of applicants for Aerospace Engineering in each institute.
• Previous Years’ IITs Cutoff for B.Tech Aerospace Engineering

You can check the previous years’ cutoff of IITs for B.Tech Aerospace Engineering for reference from the tables given below.

As far as marks are considered I think scoring above 50% of the maximum marks will guarantee a rank close to 2000 which is sufficient for getting Aerospace at IITK, IIT-KGP, IITM but maybe not IIT-B. However, don't limit your efforts to attain the required rank only. Work hard and give your best and try to rank as high as you can.

Also, I would suggest you to go for IIT Kanpur if you are interested in Aerospace as it has better facilities in this branch. But it is a well-known fact that the job prospects in Aerospace are very limited in India.

If you get a rank of more than 10000, go for IIST, Trivandrum. 

Since my sister was a student of Civil Engineering branch at IIT Bombay, I have information about the same only.

• Structural Engineering: 1–250
• Geotechnical Engineering: 250–450
• Transportation Engineering: 400-550
• Water Resources Engineering: 500
• Ocean Engineering: don’t remember

The above ranks are for General Category only. Reserved categories can expect a call at a rank higher than that mentioned above. The overall admission procedure comprises 75% weightage to GATE score and 25% weightage to written test and interview.

 It's important to note that when considering the undergraduate program at IIT Bombay, you should focus solely on the general category ranks listed below. 

For the 4-year B-Tech course, you will need to keep in mind the following ranks:

• Computer Science Engineering ~ AIR (2 - 63)
• Electrical Engineering ~ AIR (25 - 288)
• Mechanical Engineering ~ AIR (291 - 1110)
• Engineering Physics ~ AIR (309 - 1101)
• Aerospace Engineering ~ AIR (807 - 2070)
• Chemical Engineering ~ AIR (832 - 1922)
• Civil Engineering ~ AIR (1498 - 3267)
• Metallurgical Engineering and Materials Science ~ AIR (2229 - 3574)

For the 4-year B.S course, the ranks are as follows:

• Mathematics ~ AIR (128 - 1511)
• Economics ~ AIR (819 - 2176)
• Chemistry ~ AIR (2541 - 5700)

For the 5-Year B.Tech + M.Tech. (Dual Degree) Course, the ranks are as follows:

• Electrical Engineering ~ AIR (301 - 749)
• Energy Engineering with M.Tech in Energy Systems Engineering ~ AIR (1566 - 2469)
• Environmental Science and Engineering ~ AIR (3830 - 5209)
• Mechanical Engineering and M.Tech in Computer Integrated Manufacturing ~ AIR (1195 - 1571)
• Metallurgical Engineering & Materials Science ~ AIR (3688 - 4213)

Remember that these statistics are important to keep in mind when applying to IIT Bombay.