Hi Rashiqa, 

The entire concept of the best-five rule is to include the marks of 5 subjects in which you have scored the maximum marks. However, for the said course Mathematics is a compulsory subject and one language paper has to be included. So apart from mathematics and a language pape, you can choose to fill in marks of any four subject in which you have scored relatively more marks. The eligibility criteria for obtaining an admission in B.Sc Mathematics at St. Stephens is as follows: 

Candidate must have secured minimum 50% marks in Mathematics and 45% aggregate marks in Mathematics, one language and two best electives would be considered best four subjects (BFS).

Also note that Class 12th Marks only have 85 weightage, the admission process also consists of an aptitude test [5% weightage] and an interview [10% weightage]

To read more about admissions to this university, read up Careers 360 at the link given here https://www.careers360.com/colleges/st-stephens-college-delhi/bsc-mathematics-hons-course-2