As you can tell from the posts, it all depends on your sleeping arrangements. For about 5 years, I slept with my two boys in dh's and my huge bed, with dh sleeping in what was supposed to be our older son's room. (We had very little sex life even then; no suprise we're divorcing now). A one bedroom apt would have been fine then. Half the time, dh slept on the couch anyway, because he liked to watch tv to fall asleep (I need total silence and use earplugs).
Now that my oldest is 13, though, he's getting private about his body, and his phone conversations with friends, and so forth; so it's time for him to have his own room, which I now can afford with my parents helping me financially. But imho, the more kids sleep in the same room, and the longer they do it, the healthier for everyone. I love lying in the room with them at night when they go to bed, talking about all the stuff they merely grunted about after school. I love hearing them wrestling and laughing in there after I've said good night.
So I'd squish as many as you can into as few rooms as possible. Stbx has a 2bedroom apt. with the 3 kids. He sleeps in his living room on a futon while the older two get one room together, and our 3 yr old has his own room (don't even ask--he's impossible to sleep with
Remember that probably way more than half of the world's population sleeps in a room with 10 or more people, including aunts, uncles, grandparents, babies, cousins--whoever!