Can I join ISRO after B.Sc. Maths?

Yes, you can join Indian Space Research organisation after completing a Bachelor of Science in Maths but as a Technical Assistant. Because, for joining as Indian Space Research organisation Scientist (entry-level), the basic educational qualification is a Master of Science or Bachelor of technology.

For joining as ISRO Scientist (entry level), basic education qualification is M.Sc. Or B.Tech. After B.Sc. Physics & mathematics you can join as Technical Assistant, ISRO allow you option of part-time/correspondence study, after completing M.Sc.