Hi,
Amulet limits the total number of artists (not individual tracks) to around 16,000 but it's unlikely you're anywhere close to that.
Instead, I suspect a dodgy character in the album or track title is upsetting the grammer engine that builds the speech profiles to allow recognition to take place. This has tripped us up in the past, though we do a pretty good job of cleaning up MP3 tags before we let them near the speech engine these days.
Could you generate a bug report please, using Start Menu -> Programs -> Amulet Devices -> Send a Bug Report. Make sure you tick the "Attached media files" tickbox at the bottom of the window, and include a brief note referencing this discussion.
I can then check and see if we can reproduce the problem here using your song/artist info.
Thanks,
Eddy