There are some quotes from FAQ:
Do I need a separate license each time I use an item in a series?
No. You only need to purchase the item once per series.
What constitutes a series (eg TV series, webisodes, or a magazine with monthly editions)?
To be considered a series, all things in the series (eg episode, edition) must be connected, and be released within 1 year of the first installment. There is also a maximum of 52 episodes or editions within the series.
So, you need one regular license within one year for 52 videos.