Each zettel should contain one single idea. Putting multiple thoughts in a single zettel effectively creates static links. It undermines the dynamic behavior that makes the method powerful.
Zettels are discreet personal thoughts, not citations. The goal is to have captured the thought you had when you encountered the thing you are citing.
The zettelkasten method is oriented around building quality connections between thoughts (i.e. relevance maps). Links with explicit context have maximum utility.