With some sims, I let them continue having children until they get a male (or female, depending on the neighbourhood) heir. With others I let them fulfill their want of 3 or 6 kids. Mostly, though, it's about 2. Not many of my sim couples have no children at all but lately I've let them decide, and if no want rolls, they don't try.
My sims can have between 1 and 4 kids but I let them have as much as they want... till a doctor arrives in the hood.
Mayor Garance (romance/knowledge) had 10.
Now that the doctor moved in Bonneville, birth control is allowed. I'm expecting less baby making.
I haven't started playing my megahood yet (doing makeovers) but from past games and what I think i'll end up doing, I think the average will be around 3. I like families in the game, a couple kids for most Sims. Though that'll just be the average, some may have as little as 1 (maybe rarely 0 but usually at least 1) to as many as 10 or so if I decide to let some Sims do the 10 kids want thing, though that many won't be that often