It isn't so much about the number of Hubs you write, but about the number of views your content has.
The main thing to understand about earnings is that it is based on impressions.
Most visitors come from search and the vast majority of those search visitors come from Google searches.
So the best way to get traffic for your Hubs is to do SEO research before you ever start writing.
The main things to understand about SEO is that you want to be competitive.
If you do this sort of SEO research before you create a Hub, then you can get an idea of how your Hub may fair in the search results.
The main thing to do to work toward more traffic and earnings is to continue to write high quality Hubs.
The more Hubs you have, generally the more opportunity you have to find readers and earn.