Forum Discussion
In a nutshell, you can't add a ninth sim to a household manually from either the Gallery/Library or merging households or transferring sims between households via Manage Households. The game is set up for only 8 sims max. The mods that I am aware of that let you have more than 8 sims in a household will only let you have more than 8 sims in the household by having babies. IOW, you grow your household to the max size you set, in your case, 12, by having and raising babies. If you take a household where you've done this into CAS, the game will delete any sims over 8 and it will do so randomly. One mod that I know of that does allow you to have over 8 sims can get around the CAS issue by allowing you to take a household sim to CAS alone, without the rest of the household, but again, if you forget and go to CAS the normal way, sims will get deleted. Playing this can be risky and tbh, I don't advise doing it. You can create just too many problems for yourself.
Hope this helps.