usb root hub 3.0 in device manager

How many usb root hub 3.0 in device manager should you see? On one of my computers with 4 ports I see usb root hub 3.0 in device manager three times. On one of my computers with 3 ports I see usb root hub 3.0 in device manager two times. Is it always one less than how many ports you have?

Depends on your hardware.
Each USB Hub can have several USB Ports the range is probably between 1 to 4.

Thus 2 USB root hubs could be as many as 8 USB ports.

"On one of my computers with 3 ports I see usb root hub 3.0 in Device Manager two times"

That could be 3 ports on Hub 1, or 1 on Hub 1 and 2 on Hub 2 or vice versa.
Maybe the other hub is on an internal USB header.

Change the Device Manager View to "Devices by connection" may make things a bit clearer, or consult your MOBO manual.

"Is it always one less than how many ports you have?"

No, as mentioned it is the PC/Laptop designers choice as to how many USB Hubs and USB ports are provided by the hardware.

Conventionally USB ports close together on your PC box maybe on 1 hub.