There is no minimum number of hubs you need to have. The quality of your content should be really good and more people should be willing to read it. Once the traffic to your hub grows, there are better chances of earning more money, even if you have 10 good articles. Having said that, don't stop writing. The more hubs you have with a wider range of flavor, the more number of people you attract and the more money you make.
Also, checkout the "stats" for your hub. That should give you a fair idea about the earning potential of your hub.