You were correct to ask about devices themselves, not the OS.
For instance, I believe that Android can register up to 256 touches, if the hardware allows. Other OSs would be similar.
So it's really up to which touchscreen controller manufacturer is used and what they support. Most support at least three fingers.
Note 1: There's some cool new technology coming, btw, with some of the neatest options based on resistive screens. For instance, this
touchscreen with at least ten simultaneous touch capability. It can use fingers, stylii, even see light brush strokes... all at the same time.
Note 2: As you might know, Apple's particular screen controller patent is
under attack over multitouch right now.