Which course should I pursue in the banking field after completing class 12th in science?

Don't choose your career based on marks. Understand your strengths and think where it can be utilised in work-life as you are going to work for 40 years.

However, maths will be of help but other subjects can't be overlooked. Prepare accordingly.

Only maths does not play important role in the exam of banking. You must have to make good Your English, because in this competitive exam English is very tough. So, first of all, make your grammatical concept clear and after that, go for it.