Hello Media. IMO, I think it is best to focus on one website until it is stable or at least making some decent income and then slowly create other sites.
Dont go overboard writing content and creating sites. Make sure the content and sites you already have are working. For example, if you have 10 pages of content but google is only indexing your index page try to get to get the other pages indexed before you add more pages since if they arent indexed they essentially dont exist to the search engines. You want to get to the point where content you produce gets indexed and ranked quickly.
More site is not a must, that's a fact. More sites doesn't make more money, traffic makes money. This blog post should help you: So You Want To Be a Google Whore