Hey,

The answer to your question is YES.  You are eligible for applying for NDA exam as you have maths along with commerce in your 10+2.

But you can only apply for Indian Army and you cannot apply for Air Force, Naval Academy.

Now coming to second part of your question there is usually 400 seats in NDA exam and the no of student called for SSB is five times more.

But there also a criteria that it's not a necessity to fill those 400 seats. You will only be selected if you have the potential for it.

Hope that helps.

Thank you