I can handle eight, but I often try to keep it at no more than 6. For me, 8 sims can be a real great way to make time go faster because you're so focused on the day-to-day activities, that time just flies and before you know it the kiddies have all grown up. Most households with 8 sims I run like a military operation. I figure out everyone's schedules, and I keep them in a routine. Obviously I take breaks from this once in a while. I've found that when I come down from playing an 8 sim household, I temporarily feel a little lost because there's less grind in smaller households. Lol.
