usb

Image from usb3.com

A USB 3.0 port is actually seen as 2 ports to macOS: a USB 2.0 and USB 3.0
Apple choose 15 ports as the limit.USB Hubs attached to one of your USB controller’s ports have a different kind of port limit. In total, a single USB port can be split into 127 ports. This includes USB hubs attached to USB hubs