Is there a limit to how many videos I can add to my Watchlist?

No. You can add an unlimited number of videos to your Watchlist, however, only 100 videos appear at one time. Once a video is removed from your list, a video further down your list will take its place, always totaling 100.

* Keep in mind that if you have saved a video to your Watchlist and the video expires, you cannot view the video anymore.