one physical USB-A socket on the machine can read as two USB ports virtually due to that physical socket 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 (virtual) 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

someone is going to nerdsnipe me so i will add that technically its 15 registered ports per USB controller, so you can add a dongle with another USB controller and have more than 15. a mobo usually only has 1 USB controller for way more than 15 USB ports though, Apple added this arbitrary restriction for no reason since none of their devices have that many ports. so a typical PC motherboard just straight up destabilizes the system lmfao