VIDEOIO ERROR: V4L2: getting property #7 is not supported

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The CV_CAP_PROP_FRAME_COUNT will give you the number of frames in a video file. That doesn't apply for a camera (as you don't know how many frames will it capture).

I think you want to use CV_CAP_PROP_FPS which gives you the number of frames per second.

More on the VideoCapture class.