Yes , you can pursue M.Sc Mathematics after completing B.Sc in Computer Science but not all universities allow this. For this, you need to check the eligibility criteria for the particular university which you are willing to enroll in.

However, To pursue M.Sc in Computer Science after completing B.Sc in Mathematics is normal. Most people choose this.

Simply, just check the eligibility criteria of the university you are willing to enroll in and if it allows you, you may enroll in that program.