To appear for UPSC, there is no stream or discpline of subjects that you need to study per se. You can appear for UPSC with Graduation in any discipline. So there is no restriction of subjects that you need to and can choose to appear for UPSC.

UPSC Prelims has 2 papers. Paper 1 covers Current Events, History of India and Indian National Movement, Indian and World Geography, Indian Polity and Governance, Economic and   Social Development, General Issues on Environmental ecology, General Science.

Paper 2 covers Comprehension, Interpersonal skills including communication skills, logical reasoning and analytical ability, decision making and problem solving, General Mental Ability and basic numeracy numbers.

Now if you look at Paper 2 this is where your Mathematics knowledge can help you. And you also need to have a back up plan. So you can pick up the conventional B.Tech/B.Arch or take up B.Sc in Acturial Sciences, Statistics or you can also take up B.Sc Mathematics. Or you can study B.Pharmacy too. So you can take up any degree after your 12th and still be eligible for UPSC.