Need help with tags

Hi guys, I`d like to ask if you limit yourself to a small number of tags when uploading videos to YouTube or when posting on your WP blog? I've researched the topic and I found different answers so I`m not really sure what to do. Some say that it`s better to stick to few tags, some say to use plenty as long as they are related. I`m really confused as I have started two WP blogs and i`d like to get the things right from the start.
Any help is really appreciated!