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 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

Okay but what kind of application is it used for that you exceed 15 ports filled??

Off the top of my head: Sensors. E.g. in some underfunded university lab.