Hi Heeramani,
There are 7 institute participating and admitting students for IISER through SCB.But the rankings are based on which subject you want to pursue.On a personal note I would suggest you to go for IISER,Kolkata as it the oldest institute compared to others.
For further reference,I am attaching the link which would clarify all your doubts regarding IISER admission process: https://engineering.careers360.com/articles/iiser-admission
Hope it helps
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