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.