IIRC, each USB port can support up to 127 devices.
Actually, the Main Root can support 127 Devices. Devices include USB Hubs.
If there are two USB Ports on the motherboard, chances are that those are from one root.
If there are four, they might be from one root, or possibly two roots.
Example: 2 USB Ports on back of unit, plus the root, equals 3 USB Devices. Add two 7-Port Hubs gives 14 ports, and tallies 5 devices. Add fourteen 7-port hubs onto those two others gives 98 ports to use, and 19 devices used for a total of 117 devices. Two more 4-port hubs could theoretically be used to maximize the bus at 127.
You can also daisy-chain up to 5 tiers of devices in total. The root on the motherboard counts as one, so 3 hubs could be used between the motherboard and the device.
Also, the maximum length between devices is 16'. anything longer requires a powered hub to boost signal integrity.