As klanguedoc already suspects, it is not the number of hubs that earn you money, it is the total number of pageviews (and clicks on ads) you get.
Obviously, someone with a 100 hubs that receive a total of 10 views earns less than someone with 10 hubs and a 100 total views.
To get your number of views up, you might try to write hubs that cover a subject that hasn't been covered a lot already. Also, try to gain followers by participating in the community (on the forums, commenting on other's hubs, etc.).
Overall, i'd urge you to write about things that truely interest you, and read other's hubs and comment on them if they truely interest you. You should write because you like writing, and then after a while, you'll be surprised how much you've earned just by doing something you love.