I've put loads of TrueType fonts (20+) on my Gen3 to try, but it only shows me 6 when I go via the menu.
I've looked on Google and on here and followed Harry's advice here
A: Create a folder called "Fonts" (the name is case sensitive) under the "CyBook" USB drive and add your TrueType or OpenType fonts to it. If, when you add a new font, it doesn't appear in the selector, go to the "system" folder, and delete the file ".fonts.cache-1". The new fonts should now appear when the Gen3 exits USB connectivity mode.
Plug into USB port, turn reader on, delete fonts cache via PC, safely disconnect USB device on PC to write buffers, unplug from USB port, turn reader off, turn reader on again.
But it's still only showing me the same six fonts.
Before I waste any more time messing about with it, I thought I'd better check that there's not a limit of 6 fonts on the reader or anything obvious I'm missing.