There are two possible reasons.
1) You may be overloading your USB hub. We recommend
usbview to help you determine which ports are on separate hubs.
2) If any two webcams are the same model, they might both be telling Windows that they're the same webcam, which confuses OBS. On one of the duplicate webcams, do this:
* Open Device Manager.
* Expand Imaging devices.
* Locate the name of your camera in the list and right-click on it.
* Select Update Driver Software...
* Click Browse my computer for driver software.
* Click Let me pick from a list of device drivers on my computer.
* Check Show compatible hardware, and then select USB Video Device.
* Click Next and follow the on-screen instructions to update the driver.