Hi Animesh, welcome to the Atlassian Community!
When you enable SSO, you create an organization and the domain name for that organization gets verified (claimed) by the SSO provider. This means that all emails using that domain will have to use SSO to login to the Atlassian products, no matter in what (cloud) instance.
This means that you cannot exclude one Atlassian site from the others, as long as the users logging in have the same email domain.
You can, of course, add users with another email domain and they will not have to use SSO if their domain name isn't claimed.
For more information about using SSO with the Atlassian Cloud products, please take a look at this documentation
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