Under XPs implimentation of USB2.0 you are restricted to 127 devices per Host Controller.
There is no limits to how many of each class you can have on the same bus.
So in therory XP can support 127 joysticks, pedals, yokes and throttles.
Windows does number these devices in some arcane way. (My MSFFB pro always ends up as device 1)
I read some where that IL2 only supports devices 1-4.