As far as I know there is no limit on number of files file listener has to process. If all files comes at a time increasing concurrent threads in File Listener data instance would work.
Enter 1 or a small positive integer to determine the number of threads per server node this listener requestor creates when it starts. Each thread operates on a single file; multithreading has no benefit unless multiple files are available at the same time for processing.
If the node on which this listener runs has available JVM memory and CPU cycles, and multiple files are available for processing, use of two to five threads can increase throughput.