The num_cpus specifies a limit for ray on the number of concurrent tasks that it can schedule. They cannot sum to a value greater than num_cpus. It does not however place any restrictions on the actual amount of system recources the running task utilizes. You could schedule a task that you tell ray requires 1 “cpu” but actually use 400. At that point it becomes up to you to make sure you are not oversubscribing the system. In some ways num_cpus is a misnomer it is really more like max_tasks.