I think it would depend upon your sims traits, such as are they family oriented? A sim, especially both of the parents, will do a better job at raising several children over ones who aren't family oriented. Are both parents working? Can you afford a maid? To buy the toys and books that help in raising them?
You can take them to the library to read them the books, but I prefer to read them to the kids at home. Read to one and have the other reading one on his/her own.
I have a sim couple now that I gave a much longer life span for YA and for Adult (I think it is 50 days), gave for the toddler 15 days and teen 15 days. That way I could spend the needed time with the kids to give them what I feel is needed. But I am new to such type of games, so I am not good at multi-tasking so that is a factor.
Right now, my sim family has four, two newborns, one a toddler and one who has just aged up. But before "yesterday" sim time, there were four cribs, two in the kitchen and two in the bedroom.
If all goes well, and I think it will, I will have another set of twins after the youngest set age up to child. That will give me six. But, I would not think of trying it without both parents being family oriented and one parent who can stay at home and have a maid. At least not until I had played the game for quite some time and feel really confident about my ability to multi-task.
So, in answer to your question, it depends on many variable besides the desire to have kids, just like in real life.