How do you decide when enough is enough?

My sim couples usually have two kids because I become too obsessed with trying to keep the family together and in the same house...even when adults, which is not realistic. (Unless they are living in a mansion) lol. I also am bad at deciding who to play for the next legacy. Fewer children mean fewer decisions. I am playing a couple now and forcing myself to let them have like 4 kids just to see how it plays out.

Three is my magic number tbh. Two YAs move out, the other one stays home and marries/has kids. I usually then have enough room for them to work on their three, and so on and so forth. I also play all my sims, so each household goes legacy style with an heir and all that. It can be great fun!

My 100 baby matriarch is currently pregnant with triplets, which will equal 21 babies, and she's 5 days from being an adult. I'm hoping to hit 50 babies in her life time. So I don't think I know when enough is enough. Maybe when you are more ready to pull out your hair then play your save? :D :D

@ChelleJo but for your normal families how do you decide?

My sims usually have 3 or less.

For my normal families, it depends on the sims. Family oriented sims can have as many as 5 or 6 kids, while a confirmed bachelor or bachelorette might only have one (or 4, depends on how much I feel like torturing them).

Really, outside challenges, I let my sims decide basically. What kind of person they are, and what risky woohoo gives us, determines what they have. My happy place is 2-3 kids, but I'd had surprised with risky woohoo often, and I am very much a roll with the punches type player.

And if my sims roll whims to TFB, I indulge them, regardless how many kids they currently have.

I have a mod that prevents townies from generating (homeless sims I've created take up NPC slots). So, most of the time, my sims usually have 2 to 5 kids because I need to keep up the population.

That's an excellent question. I've been trying to change my play style so couples don't immediately start having kids after they get married because sims invariable always roll whims for that, and I have to teach myself to push that off. That way things aren't hectic with my sims juggling their jobs and raising a family. I have several families with 4-6 kids, and others with 1-2. Really its just a gut feeling I have, if I find myself bored of raising babies, I know I've had too many kids, but I also play rotationally so I can jump to another house (usually a single sim) and shake off some of that raising babies stress.

Usually, I say once the oldest is about 4/5 days from turning into a teen is when I stop.

Oh, you'll know when it's enough especially when you and your sim taking care of your kids start getting overwhelming and frustrating.
With another family with a sims couple of mine, finally getting the third child aged from toddler to child where they'r emore independent to do things on their own is definitely a blessing and I decided no more kids, since I wanted my sims to start living a bit more of their lives to have more time to themselves and throw parties and the like.