It has been quite some since I used Poser 6, currently using PoserPro.  There is a finite default on how many thumbnails to display but IIRC that number can be changed under general preferences.  In PoserPro, at least, the choices for 'Thumbnail Display' are 'Never collapse thumbnails' and 'Display text-only when number of folder items exceeds'.  I do recall changing the number to 500 with that second choice in Poser 6.