Hello Shaurya,
You need to include English/Hindi and economics along with two more subjects (except physical education, music etc which are not considered as core subjects by delhi university. Check the DU website for the list)in your best of four percentage. Also, you should have studied mathematics in your class 12th. However, it is not compulsory for you to add it in your best of four. Change in stream will lead to 2.5% disadvantage. If you have not studied economics in your class 12th but have mathematics, you are still eligible for the course but with a disadvantage of 2.5% in your best four.