The problem here (for me, at least) is realism. It would be realistic to limit the number of hubs, since no airline in the world would ever have more than 10 hubs. It's just not useful, and probably too expensive for any airline in the world. On the other hand, as Yuni said, airlines aren't limited to a maximum number of hubs IRL.
I wouldn't limit it at all, just let the player decide. What you can do, is make it less attractive to have a lot of hubs, by making the focus cities more expensive when there are (for example) 8 or more hubs already. This would in some way act like administration costs or something. Haven't really thought it out yet.