I converted about 100 old video tapes....are any programs that will scan the video file and create photo thumbnails at a specific interval?
By "scan the video file", this assumes you've already captured the tape to a video file, which requires some kind of playback device with a compatible digital interface to your Mac. Unfortunately video tape capture can only happen in real time. Thus if you have 100 six-hour tapes, that is 600 hours required to capture those to files.
Once the tapes exist as files there are various apps to catalog these and make thumbnails. One of the best is NeoFinder: http://www.cdfinder.de/