one physical USB-A socket on the machine can read as two USB ports virtually due to one port supporting USB 2.0 and USB 3.0 at the same time, I believe its a backwards compat thing. I think there might also be internal USB ports as well but don’t quote me on that. On mobos with 8+ physical USB A sockets it can easily get past the 16 limit unless you specifically disable the USB 2.0 port, which then might break older devices relying on that and it becomes a whole headache. this person explains it better than I can