You need to score minimum of 50 percentage and above in your graduation from a recognized board or university. The admission is taken via both the entrance exam and merit based. You need to clear the Msc entrance exams like IIT JAM, BITSAT, CAT, NEST, CUSAT, etc.

You can refer to the site below to know some of the top universities providing Msc mathematics..

https://university.careers360.com/colleges/list-of-msc-in-mathematics-universities-in-india

Also refer to the site below to know the eligibility criteria of the top bsc entrance exam ie IIT JAM..

https://university.careers360.com/articles/jam-eligibility-criteria

Best wishes. Thank you.