I have multiple projects that I want to merge together in a single project with the Single Sign-On. I have checked its possible using a single realm with multiple clients, What I need to know is it possible using multiple realms as well?
Realms are separate entities and have their own session.
If you have a single user database, you can have tiered approach, where you have a top realm, with your users, and second level realms with the applications as clients.
You can have the second level realms import users (so you can manage them per-realm, if necessary).
Second level realms will use the topmost realm as an identity provider.
But if you have a single user base, there is little value in using multiple realms, only if you need to provide group and roles management capabilities per application. Better explained: if you need one application to view some rules and other application to view other rules for the same user, you need multiple realms.
If you have multiple user databases, and they are federated (ldap or active directory), you can have multiple databases in a single realm.
Hope that helps.