(By the way, the two MegaTraveller computer games had one starport per world, located in a "startown")
I view most worlds as having a single big starport each; high-pop worlds, and/or balkanized (Gov 7) worlds, might have several spaceports, but more than one starport is rare (and, again, it happens in most cases on Gov 7 worlds). IMTU (the Solar Triumvirate), most worlds are quite newly-settled (in the last 100-200 years), and rarely had the time or money to build more than one starport, and if time or money were available, in most cases it went into improving the original starport rather than building additional ones. In the OTU, as I see it, the starport plays a political role as an extraterritorial outpost of the Imperium on that world, and as a conduit of interstellar commerce; As the Imperium (and most Traveller products) see each world as one "location", I hardly see them building more than one starport. Again, local governments may build their own, but Imperial starports get big Imperial subsidies... Which keeps them in an advantage over the local ones.
Thus, the startown is the urban growth around the starport, and is usually composed of many warehouses, of entertainment establishment aimed at offworlders, of markets, of Imperial facilities, of residential areas for starport employees, and, ofcourse, of shantytowns or slumsinhabitated by various locals who seek their fame and fortune in temp jobs at the Starport.