That is just the system catching up, leave it for a few seconds to sort it self out and they become available.I tried opening IE windows and got to about 35 at which point I couldn't open any more (got a nasty beeping sound and a useless IE window).
try system managed, windows knows best
It seems to change on the fly. At times Im limited to around 15 (explorer and internet explorer) windows, but when I do a reboot, I can usually open around 20 IE windows before it stops me from opening anymore (ie. you click on IE shortcut and nothing happens).
That is just the system catching up, leave it for a few seconds to sort it self out and they become available.