Huh, I don’t think I’ve ever seen a controller with more than 15 ports anyway. Usually looking through the device manager (or lspci) there are multiple USB controllers on a motherboard (mine has 4 for example) so theoretically you could do 60 ports. Also what can do you about chaining to hubs? Since technically the spec lets to chain up to 127 devices.

But the bigger question is, what in the world are you doing that’s requiring 15 ports.