How much should I get in her to get admission in iist
Assuming asking for B.tech IIST takes the admision through JEE Advance. As per the eligibility you should have minimum 75% in class 12th as a general category & 65% as a reserved category. You can check teh further information with the given below link.
https://engineering.careers360.com/articles/iist-btech-admission
Good Luck!