My understanding - there is no limitation on number of USB devices. There is however a limitation on the number of USB devices with axes. This limitation is 4 devices. TrackIR axes do not count as axes - it is physical mechanical axes that matter. The mouse wheel doesn't count to this limit either
So you can have up to 4 joystick/slider/throttle/pedal type devices then as many non-axis devices as you want e.g. push button controllers
So the key is to get all your movable axis requirements onto four of your devices
I went for a Leo Bodnar board because it lets you have so many axes on one device:
Device 1 Sidewinder FFB
Device 2 Saitek rudder pedal
Device 3 CH throttle
Device 4 Bodnar BU836x - all three trim axes, radiator, mixture, prop pitch, flaps
Device 5 Button/switch board (GS)
Device 6 Trackir
Device 7 mouse
Device 8 keyboard
Hope that helps!
Regards,
Martin