You can only have 8. However, if you have 7, and one of those Sims is pregnant with twins or triplets you can have more.
you can have up to 8 sims in a household
Up to 8.
You can create as many families as you want, however you can only play one household at a time, unlike Sims 2, where you can control the whole town at the same time, by going to different households.
The only way to play a different household is to make it your active one.
Nope, you can only have 8 people in a household at one time. If you want your sims to have more children, you're probably going to have to move some people out.