The :thread_policy keyword argument of op:create_poa controls the number of threads created. The value can be one of:
Use the same thread for all requests.
Use an ORB specific number of threads.
Use n threads.
Currently the "ORB specific number of threads" is 1, so this is the same as :single-thread-model.
See the Portable Object Adapter specification, available from OMG.
Developing Component Software with CORBA - 7 Aug 2017