Household of more than 8 sims in The Sims 3

Hi, to keep it simple, short, and clear, you need a mod called MasterController (available on the NRAAS site). Once installed (installation tutorial for packages > https://sims.fandom.com/fr/wiki/Contenu_personnalis%C3%A9), you look among the options in the new menu that will appear when you click anywhere, NRAAS, and you should normally find the option "More than 8 Sims".
If you can't find the option, you may need to click on the town hall > NRAAS and search among the available options.

Note that you will need to add the Sims with this same mod. You won't be able to do, for example, "Ask to move in"; you will have to use the mod to add the Sim to the family.