Manual:$wgFileCacheDepth

Details
This variable controls the depth of the subdirectory hierarchy created under to store cached pages when.

By default, two levels of intermediate directories are created. The directories are named using prefixes of the hexadecimal MD5 hash of the page title.
 * For example, if the title of the page was "Example article" (and since "Example_article" has the MD5 hash "cfeac3baf7439e4a49137bdf433f8d88"), the file in which the cached page was stored would be named:



The purpose of the subdirectory hierarchy is to spread the cached files evenly around several directories, thereby reducing the number of files in each individual directory. On some file systems, this can improve performance if the number of cached pages is large. However, if your wiki is small, or if you know your file system can handle huge directories efficiently, you might want to set to get rid of the intermediate directories entirely. (Conversely, if your wiki is huge, you could try increasing the value to spread the files over even more directories.)