Well it is debatable. First of all, I hope that there is never a video titled "Beatles Cover Song", because you would need to have the title of the song in there. The amount of tags you have should be dependent on the size of your title and the broadness of the searches you wish to rank for. For larger channels, you can rank for many different searches at one time and use lots of tags. For smaller channels however, you should focus primarily on one search and only the tags that would apply to it.
YouTube's search results update all the time. I find that after I make a change, then it takes 24-72 hours to take effect in searches. Sometimes it is more, but sometimes it is quicker. You will have to keep trying and experimenting, because there is no set formula to get it right. Also, your subscribers come when people enjoy your videos, but they cannot enjoy your videos if they cannot find them. The search engine is the primary way to find videos, so indirectly it should help you gain subscribers.