The Manual describes a 20000 track number limit for the USB player option, so that sounds enough.
I just tried a 16GB Corsair Voyager memory stick loaded with 195 Albums and almost 2500 tracks.
In directory mode, everything is shown, but the Genre, Album and Artist listings are not completely filled. From what I tested, this seems to stop a little above 1000 tracks. the rest of the tracks is then stored under "Various Genres", "Various Albums" etc..
My 4GB stick with below 1000 obviously worked OK.
I did see people report here that 16GB also works OK, but would be interested to hear from them about the Genre, Album and Artist info being incomplete.