How do you decide when enough is enough?

Usually I go with 2 or 3, every now and then I'll go for 6 but no more than that. Really just depends on how I feel and how attached I am to the family.

2 to 3 in most saves but it does depend on the family for sure. For instance I have two very involved in their career sims in brindleton bay, didn’t make sense for them to have more than one child. But another family has three because moms aspiration is the family one which requires three kids at least.

I usually plan ahead how many kids a family is going to try for. It's never more than three, with the exception of when I did the toddler mania challenge. And if a family does have three, dang it, they are going to get that Big Happy Family aspiration whether they like it or not! It's too useful. ^_^

Of course, sometimes the game messes with me and gives me triplets when I was only trying for one. >.<

4 is max but I usually stick to 2. As much as I'd like to carry on, I can't devote as much time to each one if I have lots. That's what I keep in mind to stop myself.
It also depends on the couples, some would want a big family and others would be happy with just 1 or 2. So I stop when it feels forced.

I usually only let my sims have one or two kids, as I prefer smaller families. But I do have MCCC's Risky Woohoo set to 5%, so there's always a chance they'll accidentally have more. Abductions are a possibility, too. For example, I had one sim who was only supposed to have one kid, and she ended up pregnant with twins the first time. Then she had a brief fling with a townie and ended up pregnant by him because of Risky Woohoo. Finally, she was abducted and ended up with a child from that. So one kid turned into four because the game apparently did not agree with my plans for that sim.

Generally I go with an heir and a spare. I am not really into raising a lot of kids, I'd rather play a young adult or adult.

I usually give my Sims no more than 2 kids because it's easier to manage the household. I also have attachment issues, which means that I'm not capable to choose one heir and ignore the other sibling, so having just 2 kids is better for my rotational gameplay. The only exception is with my Decades Challenge family, in which my founders had 6 kids. Boy, was that messy! It was worth it, though, I love seeing their complicated family tree.

I use MCCC, so with risky woohoo at 8% and autonomous woohoo active, I just use Try For Baby once to get an heir and let fate decide the rest. It's rare for me to end up with more than 2 unless a particular challenge calls for more.

All my characters are predetermined, I planned out family trees before I've even made the 1st generation in the game

and then it depends on the Family

For my Main Household, the King Family, I knew the last generation would be 2 kids. because this family reflects my life.
Lola had a illegitimate son, Douglas; Douglas had Trent, (Still debating if I want him to have siblings tho); Trent Marries Julia and Has Mason and Desiree

For the Jordan Family, my biggest fully developed family, they are my wealthiest, of werewolves, The first generation only having 3 kids, Bruno, Maribel and Caleb, the entire family is basically slaughtered in 1881, Caleb Survives, spends the next hundred or so years as a wolf in the ruins of his abandoned Mansion, and is rehabilitated into modern society as Caleb Jordan Jr. he has six sons, with his wife Roxanne,
Aiden, Cain and Wren (Triplets)
Orion and Zane (Twins)
Tyron (Oops We drank to much and got pregnant in Paris but we thank God for him)
But, this was because This story was a "Beauty and the Beast" type of Story Girl who longs for adventure, comes across a rundown mansion, finds a literal beast, a werewolf that almost kills her, finds out the beast is a Human, who had his world burned before his eyes, with the Help of Julia King, She restores his humanity, and helps rebuild his Family, and Pack.

The Dior Family, Usually a Small family, it was Dustin and Amara the First generation. By the way This family is guilty of murdering Caleb Jordan's original Family. it's a Vendetta because Adam Jordan, Caleb's Father Killed Dustin Dior, So Amara age 16 Taught revenge and led a raid on the Jordan mansion. and the Family became Hunters for all Magical Creatures.
Amara had a Son, Kieron; Kieron had two kids Chase and Catherine; and then Chase had a Daughter named Aria... who is dating My Character Mason.

For the Hogan family, I thought Thomas and Jerome should adopt Garrett and Flynn, one of the two whom are antagonists of my story

For the Navarro Family its pretty small. there was a lot of sleeping around though.

Anyway, Most of the Houses that play second roles to my main houses aren't that developed in generations and siblings. it's usually one or two.

For me it changes from household to household and comes natural. I don't really pre-plan things like that very much unless I find it necessary to the story I am thinking of for that household. I like lots of variety and every household to be different than the others.