Brian Slater is right...it could just be one. How much you make per hub all comes down to how searched the subject you write about is, how well you pick your keywords to rank well, how much money each click is worth with those particular keywords, and how well you promote yourself. I have over 200 hubs now and make a decent amount, nothing too fantastic but enough to keep it in my work day, but most of them were written before I had any keyword understanding at all and I did very little to enhance the hubs or promote them. The ones I posted within the last couple of months are doing much better than all but about a half a dozen of these, and about 20-30% of my earnings are still tied up in a single hub that just happened to be in demand, even though I didn't know it at the time I wrote it.
To sum up: the number doesn't matter, here quality definitely rules over quantity.