There is no definite answer. For example, Misha here at HubPages earns a lot from just a few hubs while others have written hundreds of hubs and earn the same or similar. It just depends on your strategy: do you focus on writing a lot of content or do you focus on promoting the content that you have carefully written?
You can also do both but for example, I now focus on writing content because that is what I like best. When I know more about keywords research and promoting online content I may very well change my strategy on how I earn money.