With Hardware Version 11, the supported number of root hub ports per xHCI controller is eight (four logical USB 3.0 ports and four logical USB 2.0 ports).
In the vSphere Web Client, you can add one xHCI controller and one EHCI+UHCI controller. With VMRC, you can add one virtual xHCI controller, one virtual EHCI controller, and one virtual UHCI controller per virtual machine.