On big site with quality content should be enough to make a living. Multiple sites would mean you won`t be able to focus on them so it`s not a very good idea.
But if your one big site failed or got a problem...you have a serious problem. My old professor told me "boy have everytime more than three things running..."
If you think of a great site idea, do not hesitate to try it out since it does not cost very much to run a site in this day and age. I spent three months developing a huge site with my own indexed search databases, image and video galleries, site totaled over 65 GB when completed and have not made a dime yet. Another idea site took me three hours to create and had 25 orders the first week after one evening of marketing!