According to wikipedia: The player will visit five cities over the course of the game, including Detroit, Shanghai and Montreal.
Detroit and Shanghai are both hubs, Montreal may or may not be a hub. I don't know what the other two are. People are assuming Philadelphia (if I remember correctly) is one of the cities because the concert art of the helicopter pilot apparently uses an image of it as the background.
I see. So at least two hubs. Maybe the others are for missions only.