Stock RH 6.1, XF86_SVGA server, gdm, as root (just to make sure it
wasn't perms)

from a text screen, startx -- :{1,2,3} all work fine, starting up
an X server on vt's {8,9,10} respectively.  doing the startx -- :4
to start a fifth server, the screen blanks like it's starting X, then
it dies and kicks back to the text vt, with the messages:

failed to set default font path 'unix/:-1'
Fatal server error:
could not open default font 'fixed'

Out of curiosity, I stop'd and start'd xfs and then :4 worked fine
(this will probably break fonts on :{0,1,2,3} though :)

I can't find anything in the xfs docs about a limitation to four
X servers, so I'm unsure of how to fix this.

This is on a user's machine (board designer) and he claims that the
capability to start 6 X sessions worked fine in RH 6.0 and only after
the 6.1 reinstall (fresh install, no upgrade) did it fail.