You can have a total of 10 sims per household.. Including sims and pets.
You cannot exceed 8 sims
You cannt exceed 6 pets
For example, if you wanted 6 pets you could only have 4 sims, and if you wanted 8 sims you could only have 2 pets.
Some find this annoying but I personally think it works well as I tend to have three or four sims and about three pets