Salaam Everyone.
I did bscs two years ago and I'm confused whether I should select Computer Science or International Relations. It seems like CS course is much more than IR. And If I select IR, it's gonna help me in other subjects like Current Affairs and Pakistan Affairs. But I have no idea what is IR!!
Since CS is my background I'm confident that I can score good in CS but for that I need at least 4, 5 months for study because it covers a lot subjects like Software Engineering, Programming, Web Development, Computer Architecture etc.
So Should I go for CS or IR?

In my opinion, you should go to computer science and physics because you have background and this will help you in scoring greater number than I.R. as both are science subjects. Then questions appear about overlapping topics of PA and CA , overlapped topics can easily prepared, we all studied in day to day like kashmir issue, nuclear issue, dam issue, hydropolitics and others. You may cover these topics by reading newspaper daily. Current affairs and Topics of PA are contemporanous in nature and only source of information would be newspaper and helps in eassy also.
Now you can imagine future when you keep science subjects.
You scored 160 + in both and you will also get allocated in prestigious group. There is time of 7 months for 2020. This is enough time to study.

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“Never leave that till tomorrow which you can do today.”
Benjamin Franklin

The problem is that I need to prepare 12 CS subjects to clear this Paper.

I would not recommend picking Computer Science. Although the allure of *possible* high marks is quite intoxicating but it is very risky. The 5 or 6 months which you will spend on learning just "one" subject, you'll be better off spending that time in 5 other subjects and prepare them good enough to definitely get better marks and recover the marks that you could have probably scored in Computer Science.
Also even if you have studied Computer Science (i did too) but the course is just too lengthy and simply too much. Besides learning Pol. Sci. or Int. Relations won't hurt and you might even find them interesting.
One more thing to consider is that CSS is atleast a three year commitment. Revising such lengthy course before papers each time and then on paper night will be very difficult.