[PetrusV]

There cannot be more than 7000 folders inside a folder. But there can be unlimited folders and subfolders as long as the first rule is not violated.

If save to disk was the solution I could have done the following:
{author_sort:.1}/{author}/{title} - {authors}
With the above statement 'Andrew Abernethy - The Beginning Of All Things' would have been saved as A/Andrew Abernethy - The Beginning Of All Things
However, I cannot use save to disk because I need the metadata.db (as I use COPS OPDS).

I DID manage to find a way to resolve the issue although it certainly is not pretty. I add an author for the letter of the Alphabet I wanted.
Example:
For all the authors with surname starting with A, I add first author 'A & '.
Daryl Aaron becomes A & Daryl Aaron
Andrew Abernethy becomes A & Andrew Abernethy
Kathy Abranovich becomes A & Kathy Abranovich
This way all these authors are stored under folder A, but their names remain in the database as well.

Like I said, not pretty, but it does work....