How do you decide when enough is enough?

I usually have a rough idea about how many children (if any) a couple will have. For me it all depends on the story, the characters and their personality and lifestyle. My couples usually have 1 - 3 children maximum as I think any more would make it less fun and mean some sims get kind of forgotten. The only household I'm not entirely sure on is my main one. They only have one daughter (now a toddler), who in the story was a fairy/changeling baby that was left on their doorstep as a 'gift'. They never intended to keep her, but became so attached they eventually decided to adopt her and are now a nice little supernatural family unit trying their best to be just any normal human family. I am still debating whether they should adopt an older son now with the wishing well. Everything works quite well with just one child as it allows me to devote lots of time to each sim, plus I play with ageing off. The parents are very different too - one being a good vampire who really wouldn't want the mess or stress of more children (although ironically he is the more 'maternal' of the two), and the other being a werewolf who has always wanted and would love a big happy family with lots of children. He might be able to persuade his vampire that they could give another supernatural child a normal, safe and loving home, but I'm not sure yet. Perhaps they should just adopt a furry son i.e. dog or cat instead? :)