UPSC (Union Public Service Commission) is one of the most prestigious and competitive exams in India. It is conducted in three stages - Preliminary, Mains, and Interview. The UPSC exam has a vast syllabus, and candidates need to prepare for multiple subjects.

The number of subjects in UPSC can be broadly categorized into two categories - Compulsory and Optional. Let's have a look at each of them in detail.

Compulsory Subjects:
1. General Studies Paper I - It includes topics such as Indian History, Geography, Indian Polity, Economy, Science, and Technology, Environment, and Ecology, etc.
2. General Studies Paper II - It covers topics such as Comprehension, Logical Reasoning, Analytical Ability, Decision making, Interpersonal skills, and Communication skills.
3. English Language - This paper assesses the candidate's knowledge of the English Language.
4. Essay - Candidates have to write an essay on a given topic.

Optional Subjects:
Candidates have to choose one or two subjects from the list of optional subjects, depending upon their choice and preference. The list of optional subjects includes the following:
1. Agriculture
2. Anthropology
3. Botany
4. Chemistry
5. Civil Engineering
6. Commerce and Accountancy
7. Economics
8. Electrical Engineering
9. Geography
10. Geology
11. History
12. Law
13. Management
14. Mathematics
15. Mechanical Engineering
16. Medical Science
17. Philosophy
18. Physics
19. Political Science and International Relations
20. Psychology
21. Public Administration
22. Sociology
23. Statistics
24. Zoology

Conclusion:
In conclusion, the UPSC exam consists of multiple subjects, including compulsory and optional subjects. Candidates need to prepare well for all the subjects to clear the exam with flying colors. It is essential to have a proper study plan and follow it religiously to crack the UPSC exam.