To list all the USB cameras connected to your machine you have to install a specific tool with this command:
sudo apt-get install v4l-utils
Launch this command:
v4l2-ctl --list-devices
Example
enlens@enlens-laptop:~$ v4l2-ctl --list-devices
PayCam: PayCam (usb-0000:00:14.0-1):
/dev/video2
/dev/video3
USB2.0 UVC HD Webcam: USB2.0 UV (usb-0000:00:14.0-5):
/dev/video0
/dev/video1
Why do you see 2 IDs for each camera?
There is a common bug on Ubuntu 18.04 where 2 indexes are displayed for each USB camera. When configuring the Camera source field inside the Configuration tab of the Dragonfly Web user interface please use the first index of the two.