I am in 11th Standard now. I have PCB group. I have not taken Mathematics . is it compulsory to keep maths for seeking admission in IISER.
Depends on your choice of branches.
If your looking for the Pure Sciences, then None of the IISERs mandatorialy require a math background for BS-MS admissions.
However, if you are applying for the Engineering Science (EECS and Chem. Engineering) and the Economics programme at IISER Bhopal, then you are required to have a math background in 11th and 12th.
Also, as a previous answer pointed out, the Aptitude Test has 25% questions from Mathematics. So if you are a non-math student, you would be well-advised to maybe spend the last couple of months going over a few basics from the 11th and 12th NCERT textbooks, so as to be able to score a few marks atleast in the Math section.