Can a method from a singleton object be called from multiple threads at the same time?

You can call a Singleton object method from different threads at the same time and they would not be blocked if there is no locking/ synchronization code. The threads would not wait for others to process the result and would execute the method as they would execute methods on separate objects. This is due to the fact that each thread has a separate stack and have different sets of local variables. The rest of the method just describes the process as to what needs to be done with the data which is held the variables/fields.

What you might want to take care of is if the methods on the Singleton object access any static methods or fields/variables. In that case you might need to work on synchronization part of it. You would need to ensure multi-threaded access to shared resources for the execution of the method to be reliable.

To be able to synchronize, you might need to use lock statement or other forms of thread synchronization techniques.

You might want to refer to this article from Wikipedia which provides information on C# thread local storage as well.