How many USB HIDdevices .....

I don't think there's a specific limit for HID devices in general, there may be some for specific types of devices.

Wouldn't you be better off asking in a Windows news group?

I don't know how many Windows can handle but a single USB bus is limited to 127 less the number of hubs involved. If one assumes 7-port hubs (fairly common) then that would require about 18 hubs to support just over 100 devices. How well Windows would cope polling that many devices is another matter.

Andrew

Of course most PCs have multiple independent USB controllers/ busses... The PC I'm working on right now has a dozen, and I could probably add another couple dozen with a combination of PCI and PCIe cards.

Indeed and mine has seven USB controllers in it. So it is possible to add an awful lot of USB HID devices to a single PC! The various host controllers (OHCI, UHCI, EHCI) do offload quite a lot of work from the host but I've no experience of using a really large number of USB devices under Windows to know how that will run.

It would probably be better to ask the original question in a Microsoft newsgroup.

Andrew