IEC consists of different node types. Only the Process node type can be scaled horizontally, for example, there can be several instances of the process node. The different instances do not need to be located on the same host. It's beneficial for CPU load and thread load if they are located on different machines.
There is no straightforward formula for when it's more beneficial to instantiate a new node versus increasing the number of processing threads. Internal tests have indicated that when the number of worker threads reach 60, it is preferable to start considering instantiating a new node. The number varies greatly given the underlying hardware and message types.