TMairliner, i am playing in sandbox S4B as Leerjet air and have over 50 hubs already.
It's slow going at first, especially in the earliest stages of the game.
As you fly more routes/more flights on a route, the number of connecting passengers slowly accumulates and builds over time.
Hence to take full advantage of hubs, revisit those routes periodically and keep pushing the boundaries.
Thanks Book Siberia!
I think I had in my mind that you're limited in hubs to just a couple. I'll re-think what I'm doing.