I know of no limits to the number of X clients you can have running at once. I'd presume it'd be limited only by available memory; computer labs with terminals can have 40 or 50 X clients at a time, and probably many more than that in some cases.
I'm a little hazy on what exactly constitutes a client to the X server, though. I think Gnome or KDE would be considered as such, though, since they are making requests to the X server for displaying things. The XFree86 Howto
has a little section on how X clients are run, and it looks like any program doing display on the X server would be considered an X client.
edit: Ah, I see what you're asking... whether there's a limit on the number of clients that an X server can serve. The answer is... probably not