FME Engines connect to an FME Server Core and process requests from the Core. Adding FME Engines increases the Core's ability to perform concurrent processing of requests.
The number of licensed FME Engines you can add is limited only by the host's CPU and memory resources, which constrain the maximum concurrent request throughput.
By default, FME Server is configured to start two FME Engines, if two licenses are available. If only one license is available, only one FME Engine starts.