I have a lot of video devices in my /dev folder (e.g. video1, video2, ..., video9) and one /dev/video which
with current OpenCV 3.2.0 you can create a new capture like this:
cv::VideoCapture cap("/dev/video20", cv::CAP_V4L);
its one of the additional constructors:
VideoCapture (const String &filename, int apiPreference)
Open video file or a capturing device or a IP video stream for video capturing with API Preference.
this is also possible with the open function:
cap.open("/dev/video20", cv::CAP_V4L);
i have tested this successfully under Kubuntu 16.10 with self compiled openCV from current git master.