I enjoy those random table results that seem beyond all comprehension. I don't find the food issue a major one at all or that the enitre planet is necessarily covered in city at Pop 12.
Consider the following,
(1) Rome at it's height had about 1 million inhabitants. That's at waht TL2-3. Based on a myopic view of what medievil Europeans could do, and a handful of internet facts, such a feat is impossible. Yet the Romans did it for decades if not centuries.
(2) Consider that in the last century we have increased crop yields by about 100 fold. The population of the planet we have now would be considered unthinkable only 200 years ago (is that TL 4?).
(3) Who says the food is good? Likely it will all be some form of myconoid substance balanced in nutrients but grown in vats, add energy and watch it grow.
(4) Who says the planet is covered with city? Hong Kong only goes up a couple hundred stories max. and then not everywhere. With fusion power, anti-gravity, and the advanced materials at TL 12; isn't the ski the limit, or digging deep down?
(5) Speaking of the sky, I thought pop might refer to system population with just the primary world noted. But you could imagine a band in geosync orb about the planet. How many people could it hold? My guess is a fair amount, double digit billions.
(6) Your not going to be detailing the spaceport for a system like this. You'll take it in generalities, there will be thousand of ports, hundreds of thousands of ships. This system, even with dedicated food vats is going to be sucking in resources from all the systems around, and it's prime export (people) will dominate the sector. It is a mini-Imperium to itself, the size of an empire comprised of a hundred "worlds" of pop 10 billion. A balakanized form of government in this system can take on a whole new meaning.
(7) A pop 12 system could make for an excellent setting where FTL travel is not as easy or the universe is hard where habitable planets are few and far between. You don't need countless star systems for variety when one system alone is a sector in itself.