This is within the General Settings -> System menu
This sub menu relates to limits in the Rockbox operating system.
Max Entries in File Browser.
This setting controls the limit on the number of files that you can put in any particular directory in the file browser. You can configure the size to be between 50 and 10,000 files in steps of 50. The default is 400. Higher values will shorten the music buffer, so you should increase this...