I had commerce with maths in 12th,now I want to do ba prog eco+maths from DU, will there be disadvantage in my best of 4
No you won't be at a disadvantage, unless Mathematics and Economics, the subjects you want, aren't in your top 4. DU will then give you a percent reduction, which generally according to trends is seen to be around 2%, which can hamper your chances. So if you apply, do include Math and Eco in your top 4
As per the guidelines of Delhi University, the calculation of your best four subjects' percentage should include the three major subjects (including the subject you are interested to pursue further) and one language subject.
If you have economics and Maths in your intermediate and include it in your best four percentage then you will not be disadvantaged.
But not having these major subjects as your course in intermediate may give you a disadvantage of about 2% in the percentage.
Hope that helps!:)