As you have completed your B.Tech in Computer Science, you are eligible for MSc in Mathematics.

Generally eligibility required at least 55-60 percentages in the bachelor’s degree program, to be considered eligible for the course.
Applicants with B.Sc. in Maths with at least of 55% marks, usually, do not need to qualify entrance examination in most of the institutes offering the course.
Also as you have done B.tech, it depends your academic performance and combination of subjects you studied
Some institutes offering M.Sc. Mathematics are
Delhi University
Banaras Hindu University
Osmania University
University of Madras
Annamalai University
Andhra University
University of Mumbai
Chaudhary Charan Singh University
Dr BR Ambedkar Open University
Guru Ghasidas University
Nalanda Open University
Sri Venkateswara College, Delhi
St. Stephen’s College, Delhi
Ramjas College, Delhi
Miranda House College, Delhi
Hindu College,Delhi, Delhi
Hans Raj College, Delhi
Dyal Singh College, Delhi
Meenakshi College for Women
Dr.Ambedkar Govt.Arts College, Chennai
Pune University, Pune
Mumbai University, Mumbai
Madras Institute of Technology, Chennai
ISM, Dhanbad
BITS, Pilani
MANIT, Bhopal
MNIT, Jaipur
MNNIT, Allahabad