Can there be multiple threads in one App domain?

Yes. The .NET Framework further subdivides an operating system process into lightweight managed subprocesses, called application domains.

One or more managed threads (represented by System.Threading.Thread) can run in one or any number of application domains within the same managed process. Although each application domain is started with a single thread, code in that application domain can create additional application domains and additional threads.

The result is that a managed thread can move freely between application domains inside the same managed process; you might have only one thread moving among several application domains. For more Information see - http://msdn2.microsoft.com/en-us/library/6kac2kdh.aspx