The file consumer is single threaded, which most consumers are. Some have special options for concurrency such as SEDA or JMS.
The threads EIP allows you to leverage concurrency with a backing thread pool.
Read some details at
If you have a copy of the Camel in Action book then chapter 10 covers this in much more details.
And the thread pool which are used are from the JDK. So there should be plenty of docs and books out there about them. The JDK thread pools is not very aggressive, and that is why (1, 10) uses 1 thread, and only grows when the worker queue is full. You can configure the worker queue to a lower size.