They can be worn on their own or combined depending on the occasion and on the type of look you are striving for. In addition, there will always be rings to support each personal aesthetic and style.
In my humble opinion, an outfit cannot be fully completed if not even a single ring is worn.
If you would like to style more than two together always be extremely careful as 'the more, the better' is not always the case.
Keep it classy, keep it elegant but most importantly keep it You.