Hey,
The admission process in St.Stephens is based on composite merit,which comprises Class 12 marks:85 per cent weightage,Aptitude Test:5 per cent weightage, Interview:10 per cent weightage.
To pursue BA Economics at St. Stephens a candidate must have studied Mathematics in 12th and should have the minimum prescribed marks in Mathematics. However,it is not compulsory to add Mathematics in marks of  Best of Four Subjects.
Since you did not have Mathematics in your 12th then you can't get admission in BA Economics at St. Stephens.
For more information follow the link given below:https://www.careers360.com/colleges/st-stephens-college-delhi/ba-economics-hons-course-2
Hope this helps.
Best of luck.