The eligibility criteria as given in the information brochure of ISI is as follows:

In order to be eligible for admission to this programme, an applicant must have successfully completed 10+2 years of Higher Secondary Education (or its equivalent) with Mathematics and English as subjects.

No particular percentage of marks  (in 12th Boards) is necessary to get admission in ISI (However, to qualify the examination is mandatory).

For more information regarding ISI Admissions, kindly use the link provided below:

https://www.isical.ac.in/~admission/