Roup is the location for Foodrunner, the Amber Zone in JTAS #5. No city names are mentioned, though it does say the cities are all perched on island tops and very very crowded. There may not be all that many of them.
Lessee, just for fun - 'cause I'm strange that way:
Roup's a size 7, 7000 mile diameter. Surface area 153,860,000 square miles. Let's say it's 99% covered in water: 1,538,600 square miles. 3+ billion souls on ~1.5 million square miles, average 2000 bodies per square mile. That's actually not terribly bad. By comparison, Oahu's population density's around 1600 bodies per square mile.
You could easily get away with quadrupling the population density or more - or halving it, got some leeway there, even as a water world. However, the Amber Zone's pretty clear about Roup suffering from "massive overpopulation". So let's look at another example: Rio de Janeiro packs 6 million people into a 486 square mile area, for a population density over 12,000 per square mile, and it's as famous for its crowded slums as its scenic beaches. If we go that way, we're saying Roup's only got about 250 thousand square miles of land surface, maybe less - only 1/6 of a percent of the surface is land.
Still, that's enough room for several hundred Rio's. Place could look like the U.S eastern seaboard, city after city after city butted up against each other without a real way of telling when you've left one and entered another except for the "Welcome" signs.
Aside: The fishing industry needs to bring in 3 to 4 million TONS of food a day just to provide minimum calories. That's about ten times our harvest rate on Earth right now. We've got about half a billion people, counting spouses and children, who depend on fishing-related jobs - either fishing or processing it or transporting it or selling it, etc. So, basically, almost every working person on Roup is working directly or indirectly in its fishing industry. I'm presuming their fishing industry's also bringing in plant life or some other source of vitamins, 'cause just fish isn't going to keep them all alive. And, if their oceans are like ours, they're getting pretty close to a Malthusian solution to their problem.