The MegaTraveller World Builder's Handbook had planets covered in primary and many secondary starports and spaceports depending on the number of cities (can be a huge number) generated and the tech level. Also add in Orbital Cities.
It seems, especially in Classic and the sister Mongoose Traveller, the Highport and Downport are a mated pair with Downports often existing first and Highports added with necessity. What we aren't hearing, and Condottiere mentions, are spaceports which are a starport's world wide network for transport of goods and sentients to and from the central hub. I always considered they existed and were a transparent system in the game that strives to keep things simple. Spaceports would serve major cities. They won't be in every city. That's the job of planetary land, sea and air transportation networks.