If you want to go for the ISRO then right after your BSc there are less chance to go for it. You need to have atleast a MSc in order to get into ISRO as for most of the years the notifications are released for the MSc and BTech/Mtech candidates. So if you wish to go for ISRO then try it after MSc. You can do MSc in Applied Mathematics as there are much need of the field in the research organization as ISRO. To get a job at ISRO you can apply for GATE Mathematics or you can fill the ICRB examination form.

To know more about the career in Mathematics visit the link below:

https://www.careers360.com/courses/mathematics-course

I hope this answer helps. All the very best for your future endeavors!