Hi,

As per the official website, you are eligible for admission in Chennai Mathematical Institute with B.Sc. in Mathematics.

The official website states,

Eligibility for Admission

• B.Sc. (Hons.) Mathematics and Computer Science: 12th standard or equivalent .
• B.Sc. (Hons.) Mathematics and Physics: 12th standard or equivalent .

So you can definitely apply for admission in CMI.

For all programmes other than Ph.D. Physics, applicants will have to take an entrance examination.

For more details:

https://www.cmi.ac.in/admissions/#:~:text=Eligibility%20for%20Admission&text=standard%20or%20equivalent.-,B.Sc.,a%20strong%20background%20in%20mathematics.