There's no real set amount. It's kind of a preference really. Most of the time if the clips are 30 seconds then 4-6 should be fine but 6-8 to be on the safe side maybe. The bottom line is if the site is good, offers good content and is not full of ads or tricky links then you'll get listed for sure no matter how many videos for the most part.
Any video sites I've done have about 12-16 clips but they are the 15 second ones so that's different.
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