How many sims can you handle at once in a household?

Geez, I don't actually know. Probably only one? I prefer to play with a full house though, and can usually handle that fairly well.

Depends on the ages. The biggest games I have ever had included;
Mum and Dad (Adults)
Twin boy and girl(young adults) Athlete, Painter
Teenage boy Musician
Boy (Child) Gamer
Another boy(Toddler) Not sure yet.

I started the game with just the mum and she got married and had 5 kids.

Majority of my current rotation families are 8 sims per household. Every new family in my game expands to 8 most of the times. If not, they get pets :) I like playing large households.

I, too, have discovered four Sims is my absolute limit.

For me the secret in handling (wrangling) this many Sims is to have them all at full autonomy. Then I can focus on the ones I need to focus on the most. Also, use the pause button, it is your friend! Queue up actions for those who won't or aren't doing what they should be doing, homework, work tasks, laundry, et al, and then un-pause. I'm never happier than when the lot is bulging with Sims. I also have walked down the dark path to using the Make Happy Cheat more often than I care to admit. The Sims' needs just decline so very rapidly in this game, it's almost a joke. And, yes, I do have a mod or two or three. ;)

Five. My most recent couple had....triplets. I almost reset the game but decided to go with it...man, it's a hell. taking care of 3 toddlers running around the house with two working parents it's almost impossible.

1-4 sims is about my limit. In rotational play I won't go over 4 and even then I have to limit my sessions. If I play larger households there is usually only 1-2 sims that are important in that situation... so I'm not really managing the rest of them regularly.

I can manage a full household but I make full use of the pause button

This will most likely be the MOST I'll run in any one household.

Since Sims don't cancel their actions (when you "X" them out) and go ahead with it anyway, I can't imagine punishing myself by magnifying that aggravation by 8 (full house) or however many you can play! Until they fix it, I think 4 is my limit!

8 or more. I love playing large families with lots of children and toddlers. :) I always have autonomy off as well. Its really fun to manage the needs of a lot of sims.