Hello Gagan,

Yes, you can count Maths in Best 4 for Economics Honours in University of Delhi. 

For Economics Honours in University of Delhi, it is mandatory for you to include Economics and one subject (for you - English) and any two other subjects. Mathematics is mandatory as a subject for Economics Honours, while you may or may not include it in best 4.

Hope this helps you.

Please do tell in case of further queries.