Port: something you can pug a cable into
Interface: some silicon the multiplexer connects to one or more ports.
Type lsusb (or whetever the MS equivalent is) and the result starts something like this:
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
So, this computer has one USB2 interface, and 4 USB1 interfaces. When you connect USB1 devices to ports, each one gets assigned a USB1 interface by the port multiplexer until there are none left, then any further USB1 devices share the USB2 interface, trashing its bandwidth. Likewise, USB2 devices get assigned there own USB2 interface until there are none left, and then they have to share the same interface.
USB3 is a bit more troublesome. Computers come with separate USB2 and USB3 ports. I do not know if the multiplexer can assign a USB2 interface to a USB3 port. USB3 devices understand USB2, and will work slowly on USB2 ports. USB3 ports can speak USB2 or 1 to slow devices. A modern machine may have a few USB2 interfaces, but only one USB3 interface - even if it has two or even three USB3 ports. USB3 eats up to 10Gb/s per interface (5Gb/s x full duplex), regardless of the number of ports. If the south bridge is limited to 20Gb/s, I can see why people are not rushing to release chips with two or more USB3 interfaces.