Powered hubs should be used. As more generators are used, the computer USB output may not have adequate power output to maintain the integrity of the USB communications to multiple separate outputs.
USB 2.0 speed is the speed used. At least 4 ports, a few more are generally desirable.
USB 3.0 should be backward compatible but will be of no additional benefit, so no reason to incur additional cost.
Don't forget to change the USB power settings on the computer and turn off the "allow computer to turn if this device to save power" option. It's a battery saver option.
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