Hi Abdul,
For becoming a Scientist at ISRO you should need M.Tech or M.S.C in physics, chemistry and mathematics.
With a BSc degree,you can only become a technical assistant (TA) . So after doing MSc in mathematics you can join as a Scientist at ISRO  when they open the the posts for it.
you can check the career oppurtunities at their official website the link is provided below
https://www.isro.gov.in/careers
All the best for your future. Hope you get it, Thank you