Maximum and default size -

https://pdn.pega.com/community/product-support/question/file-listener-file-size-limit

Altering size - https://pdn.pega.com/community/product-support/question/max-file-upload-size-using-dynamic-system-setting

Concurrent processing of files through file listener,

There isn't any limitations on number of files which file listener could process.

Though, in a situation where multiple or concurrent files arrives, threading will be leverage for serving at the cost of performance directly proportional to number of files.


Concurrent Threads

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.

Reference-

Detailed information related with file listener:- https://pdn.pega.com/sites/pdn.pega.com/files/help_v72/procomhelpmain.htm#data-/data-admin-/data-admin-connect-/data-admin-connect-filelistener/properties.htm

Hope it answers most of your queries.