Node.js has been designed to be single threaded. So for all practical purposes, applications that launch with Node will run on a single thread.
However, Node.js itself runs multi-threaded. I/O operations and the like will run from a thread pool. Further will any instance of a node application run on a different thread, therefore to run multi-threaded applications one launches multiple instances.
Understanding the Event Loop is important to understand how and why to use multiple threads.