http://www.popularmechanics.com/how_to_central/technology/2415241.htmlQ: How many USB devices can I hook up to my PC at one time? Is there a limit?
A: At first the answer seems pretty simple: 127. That's the number of devices, including USB hubs, that a single USB host adapter--the device that links USB devices to your computer's motherboard--can talk to at one time. Of course, the Universal Serial Bus shares its bandwidth between all connected devices--12 megabits per second for USB 1.0, 480Mbps for USB 2.0. So while you might be able to connect, say, 127 USB hard drives, trying to access them all at once would be a disaster, bandwidth-wise. There just wouldn't be enough speed to go around.
Plus, since the USB uses your computer's CPU to do some of its number crunching, each device would take its toll there as well, slowing down your machine.
And 127 devices isn't even the theoretical maximum. It is the limit "per host adapter," which includes all the requisite chips and hardware that makes your USB ports work in the first place. Windows doesn't limit you to just one host adapter, and installing a new one is as simple as plugging in a new USB host PCI card. Each new USB host controller can support another 127 devices, meaning you could hypothetically connect 1000 USB devices if you had enough PCI slots. And since many motherboards have multiple host controllers, the theoretical ceiling for connected devices is unlimited.
In addition to processing power, most USB devices also draw electrical current, but most USB hubs pull their own power from the wall to avoid overloading the computer's USB ports. (In fact, the host controller should automatically prevent USB devices from drawing more power than they are supposed to.) That said, we find it unlikely that even 127 devices could be used at once without seriously slowing down your system. But it shouldn't do any long-term damage to try.
