Add as many tags as you can. It will make your videos way easier to find which means more views which means more subscribers which means more fame and so on. Your welcome SeanFace101 :D.
You should try to put in as many as you can, but you need to make sure they are related to the video, so yes, its better to have "quality" tags over a "quantity" of tags.
Some are single words some are 2 word tags some 3. Whatever i have in tittle and description. Whatever relates to my video. Whatever i would search myself to find a video like that.
Cool :D

It would be good if there was a tool of some sort that you can see what tags / keywords are searched more :p Like the way Google Adwords does for Google searches :p
I just recently opened an account with TubeBuddy and have spoke about it on here but haven't got around to trying all the things on it with any my videos yet :p So that's good to know that it does it on there then :D