First of all, congratulations for choosing law as a subject for your career, hope you do wonders. Now, coming to your question, There is no specific rank which will guarantee your ticket to Narsee Monjee Institute of Management Studies.
Selection Criteria: They select on basis of 12th Board marks and All India CLAT ranking. On the basis of these two parameters, they allot you a merit number. The person with low merit number are being selected until the seats are being filled up.
Therefore, there may be equal chances of a person having
- 1) AIR 1500 and 90% 12th board marks and
- 2) AIR 2000 and 94% 12th board marks.
Since the college is at its inception and its first batch are yet to graduate, you need to have minimum AIR of 1500, as people above this rank prefer NLUs. But you can't be too sure for that, even getting a rank of 3000 will allow you admission, It will ultimately depend upon the ranking of applicants and the number of seats in the batch.
Hope this helps.
All the best.