Hello Aditya, since you are aspiring to be get into Grant Medical College and Sir JJ Hospitals,  Mumbai check the previous year trends of maximum cut offs as given below:
2018 - 1122 rank
2017 - 1018 rank
2016 - 408 rank

Therefore try to secure a rank higher than these in NEET, as they are the last cut offs.
The minimum qualifying percentile for the Unreserved category in NEET 2019 is 50th percentile.
All the information regarding calculating percentile and previous year cut offs are given below :
https://www.google.com/amp/s/medicine.careers360.com/articles/neet-cut-off/amp

Also the eligibility criteria for NEET UG is as follows(for your class 12 marks):
1. You must have secured 50% in your 10+2 examination with Physics, Chemistry, Biology/Biotechnology and English as your core subjects (in both class 11 and 12)
2. You must be an Indian citizen.
3. Minimum age limit is 17 and maximum age limit for Unreserved category is 25 and more.
4. And you can have any number of attempts.

To know more about eligibility, check the link below:
https://www.google.com/amp/s/medicine.careers360.com/articles/neet-eligibility-criteria/amp

All the best!
I hope this helps.