The right number of YouTube tags for a video is the number of appropriate tags that are related to the video; beyond that it may have too many. There is not a specific number limit on the amount of tags a YouTube video can have and Google requests that users keep it to a "reasonable number." YouTube will penalize videos that have unrelated or repetitive tags because it classifies them as spam.
Video of the Day
If YouTube determines that a video's tags or other metadata are misleading, it will remove the video and count a strike against the uploader's account. YouTube uses metadata to provide context about videos to search engines. Providing inappropriate metadata may cause irrelevant videos to show up in search results, which would hurt Google's search engine business and credibility. Google looks at YouTube videos for irrelevant tags, but users can also report misleading tags by flagging a video.
A Tagging Example
Let's say you create a new video of your cat bobbing its head to a song you like. For this video, tags such as "cat," "bobble head," "nod" and the name of the artist or song would all make acceptable tags. Tags that would be considered spam include the names of other popular artists or songs, names of famous people unrelated to the song and popular items that are not related to the video, such as a tag for "Ferrari" or "Rolex."