As Adric says, Mezzmo does not have any limits of files/folders/playlists.

The problem may be the amount of data being sent to the Sony BDP, rather than the actual number of folders. The Sony BDP may have a limit on the number of bytes that it can receive in one go, and your huge list of folders is exceeding this byte limit. Serviio is showing a few more folders than Mezzmo since it may not be sending as much meta data information about each folder as Mezzmo does. We'll take a look at this issue with our Sony BDP and see if we can improve the Mezzmo device profile for it.