I mostly play with one sim trying to find love, and when he/her does that they often down have kids, I just really don't like the baby/toddler stages as they need attention all the time and the house gets littered with bottles.
@VanceEvans I agree with you I don't care for the toddler stage but I love the stage when they become a child I have a hard time letting them go to the next stage.
I guess I love the child stage because as a child it was one of my most joyful time.
"Nothing in life is a Surprise it just happen to come your way at the time".
I mostly go by wants, although sometimes I will wait and see if the sim keeps the want for a baby a while if I think they should be done with children, and I'll occasionally arrange a pregnancy for story reasons that isn't particularly looked for.
In practice it tends to mean family sims have many children (Ophelia and Johnny Nigmos-Smith are currently expecting their ninth, and thanks to a mid-life crisis Peter and Samantha Ottomas have 6), sims with family secondary have three or so, and other sims often just have the one (Have a Baby is a common want a couple of days after getting married) while romance sims rarely have any.
About a third of my sims never have children at all. Most of the rest have only one child, unless twins are conceived. I leave twin pregnancy up to the game rather than using cheesecake or forcetwins. Occasionally, for variety, I'll let a sim couple have a bigger family, but my idea of "bigger" is 3 or 4 children. I use aspirations and LTWs to guide my decision making. Sometimes, a story reason emerges that would result in another child or two being born.