Hello

There is no restrictions when it comes to choosing optional in UPSC, as in you're doing graduation in humanities you can definitely go for maths as optional, to provide you example in the past many toppers such as coming from engineering background have chosen humanities subjects as optional, and maths is pretty good options regarding this, however you have to remember one thing that the syllabus of optional subjects in nothing less that of graduation level, you have studied maths till intermediate, if you chose to go for optional definitely you have to put in lot more efforts, maths is definitely a good and scoring subjects as long as you are clear with the concepts, and can bring the same into practice, so you have to judge things on your own, for instance you may find maths interesting, but let's say if you choose optional amidst the subject in your graduation, you will find it comparatively easier to deal with the same, since you will already be well acquainted with the syllabus, and for subjects like history, polity, geography, economics etc there is considerable overlap with general studies and optional syllabus, so you have to weigh the pros and cons, as I said maths is definitely a scoring subject, as long as your fundamentals are clear, one thing you can do is go through previous year papers at the official website of UPSC, this will help you know where you stand in terms of knowledge and how much preparation is needed, practice of this along with mock papers on a daily basis is essential regarding this to boost your speed, calculation and time management skills.