See for most of the cases, BSc Mathematics won't be eligible for the MSc Computer Science as most universities prefer to get students from BSc Computer Science and as a result they set the same as the eligibility criteria. Now DU and someother universities provides admission to the MSc course to all those who have a BSc degree with the relevant subject. So if you have done Mathematics BSc then also you can appear for the entrance and can get admission to the MSc course in computer science.

To know more about DU PG admission visit the link below:

https://university.careers360.com/articles/du-pg-admission

To know more about MSc computer Science visit the link below:

https://www.careers360.com/courses/msc-in-computer-science

I hope this answer helps. All the very best for your future endeavors!