Hii Manikanta!
The admissions to Ramjas college, Delhi are in both ways entrance based as well as merit based ( class 12th percentage ). It varies according to different courses. Bsc mathematics ( Hons. ) is merit based admissions. You should have scored a minimum of 60% in mathematics class 12th, to be eligible for the admissions. You can get more details about the same on the college's official website. Hope this helps.
Good luck!