Amar, UPSC is a 3 stage   selection process involving Prelims, Mains and Interview which tests your personality test. Prelims has 2 Papers namely General Studies and CSAT.

General Studies has Current Events, History of India and Indian National Movement, Indian and World Geography, Indian Polity and Governance, Economic and Social Development, Environmental Ecology, Biodiversity and Climate Change and General Science.

CSAT has Comprehension, Interpersonal skills including communication skills, Logical Reasoning and Analytical ability, Decision making and problem solving, General Mental Ability, Basic numeracy and Data Interpretation.

If you look at the syllabus, Commerce is not there in the Prelims. But yes it will help you when you select your Optional subject. So when you are choosing a stream to be qualified for UPSC, you should also have a Plan B in mind. You should select a course that interests you, the material is easily available and that subject that you choose has had Candidates who topped in the previous years. So that way you know you have chosen the right subject. And also the stream you select should help you have a career if things with UPSC don't go as planed. So B.Com (//B.Com) LLB is a nice option between B,Com LLB and B.Com (//B.Com) Hons.