St.Xaviers, Mumbai only gives you a B.A Economics (HONS) degree. You can apply to it after you get your 12th grade marks, from certain respective boards.

However, if you're applying for St.Xaviers, Kolkata, you get a BSc Economics degree. You can apply for it with WB results when out. (WB - West Bengal) 

Hello Shweta, 

since St. xavier's Mumbai is affiliated to university of mumbai, so the forms will be out according to the dates of the University of Mumbai only.

https://university.careers360.com/articles/mumbai-university-important-dates

the above link attached will help you regarding the important dates of the forms and all other information.

hope this was helpful for you.