1 morale = 1 population 100% approval. The terminology is a little sloppy, sometimes it talks about morale and sometime approval. Anyway, starting with nothing but a civilization capital, which gives 6 morale, you can keep a population of 6 100% happy. If your population gets higher than that, your approval goes down. If you add an entertainment center, +1 morale, you can now keep 7 population 100% happy. Put that entertainment center next to an ideology building that gives +2 approval as an adjacency bonus, your entertainment center is now +3 and you can have a population of 9 at 100% approval. Upgrade that entertainment center to an entertainment network (an additional +1) and you can now have 10 population at 100% approval.
The approval tooltip shows morale/population, from it you can tell where you are and how much more you need to keep your population happy.
