YouTube too many playlists

The limit is 5,000. Once you hit this limit using the API you receive the following response:

{ "error": { "errors": [ { "domain": "youtube.playlistItem", "reason": "playlistContainsMaximumNumberOfVideos", "message": "Playlist contains maximum number of items." } ], "code": 403, "message": "Playlist contains maximum number of items." } }

Attempting to add more videos via the web page does... nothing (at time of writing).

You can simply open the playlist and look at the number of items, or query via the API.