Number of threads in Oracle XE 18c
From the Installation Guide ( here )


  Oracle Database XE limits itself automatically to two cores for processing. For example, on a computer with 2 dual-core CPUs (four cores), if a large number of database clients try to simultaneously execute CPU-intensive queries, then Oracle Database XE will process the queries at the rate of just two cores even if more CPU capacity is available. 


I would image "large number" means ">2".