How many hours a student should study to score 98% in boards for commerce stream...??

That’s a really illogical way to approach your goal. The goal should not be time-based in this case. It should be content-based. Although consistency almost always beats cramped learning, devoting a certain number of hours every day is neither a viable goal nor should it be a desirable goal. There’d be days where you wouldn’t want to study at all and there’d be days where you’d want to study for a crazy amount of time.

To be completely honest, no person here can tell you how to study to get 98% even if there are people who got that score. Why? Because they are not you.

As someone who scored 96% in 2015 in the same field as yours, and someone who taught students for 2 years, here are some useful pointers I can give:

1. Break down your goal: If you have 12 months to achieve 98%, break it down into 4 parts, each with its own goal, and gradually increase the difficulty of your goal. Mine was attempting 1st term paper without leaving any topic and finding out what my major problem areas were while scoring around 90%. I kept on increasing that eventually.
2. Invest your time in learning Active-Recall: It’s one of the best ways to study. Search it up on YouTube and you’d find tons of people who can teach you. Use that method to learn and revise your content.
3. Invest in spacing your content: Spacing is the practice of revising your content frequently over a period of time so that you don’t forget it. It’s way way better than all other ways of revision. Invest your time into using AnkiApp and you’d love what it can do.
4. Engage in rigorous Feedback: No learning is possible unless you seek feedback from experts. Those could be your teachers or ‘smart’ friends. Ask them questions like, “What did I do wrong?” “How would you have answered this question?” “What can I do to improve?”
5. Take care of your mental well-being: Meditating and writing journal are some of the best ways of doing that and trust me when I say they work. Also, these activities hardly take about 20 minutes of the day and can be done anytime.