There is a hard limit [which varies with the device [32, 64, 128 or 512]] of how many files can be in the 'root' directory. [Inherited from the CP/M FAT file system]. Some of these files may be 'hidden' from normal directory listings.

If your player can locate and play mp3s that are in subdirectories, then you can bypass the root directory limitation. The number of files in subdirectories is only limited by the capacity of the device.

This file system limitation is one of the reasons that the standard layout for digital camera memory cards does not place any photos in the card's root directory.