Yes, you are eligible for the MSc in Mathematics after completing B.Tech in Computer Science, M.sc in Mathematics course required Bachelor degree course in any discipline with Mathematics as a Main subject in 12th Standard and Bachelor degree course. It will provide you good employment opportunities.

As per your request here I am giving you the list of top colleges offering M.sc in Mathematics in India:

IIT Kharagpur
IIT Bombay
IIT Delhi
IIT Guwahati
IIT Kanpur
IIT Madras
NIT Warangal
IIT Roorkee
NIT Jalandhar
NIT Agartala
M E S College of Arts, Commerce & Science
Meenakshi College for Women
Ethiraj College for Women
Dr.Ambedkar Govt.Arts College Chennai
Quaid-e-Millath Government College for Women
Sri Venkateswara College
St. Stephen’s College
Indraprastha College for Women
Hindu College
Hans Raj College
Ramjas College
Miranda House College
Dyal Singh College
Deshbandhu College
Acharya Narendra Dev College
Madras Institute of Technology
ISM Dhanbad
BITS Pilani
MANIT Bhopal
MNIT Jaipur
MNNIT Allahabad
College of Engineering Guindy