The Restful Engine is an ASP.net application. Requests are handled by threads from the threads pool. IIS manages that pool and you can configure it to meet your needs.
Is a single port multithreaded?
Does the RESTful engine server need to be configured to handle and number of different ports, one for each thread?
The Restful Engine doesn't create threads to handle incoming requests. However it creates a background thread for processing an asynchronous job. This way IIS returns the request handler thread to the pool as quickly as possible.
A single port should be sufficient to handle all incoming requests simultaneously. If it's not, then you'll need to adjust the IIS threads pool configuration. As a reference you could take a look at the answer in the following stackoverflow.com thread