Any way to have more than 8 sims in a household?

This setting allows overriding the default EA maximum household size of 8 to another value, up to 25. Note that when you have more than 8 Sims in a household, there are some in-game issues dealing with CAS. You have to use 'Modify in CAS' for the Sims that are after the first 8 in the household from the 'Sims Commands' menu. When a new update from EA comes-out, if you try to use the update when MCCC isn't updated, it could cause problems dealing with these households. This is the same as any household size change mod, but I just wanted to make sure it was noted here that these things should be kept in mind.

If my household size is set to above 8, how do I edit Sims in CAS so the game doesn’t delete all the Sims past the 8th in the household?

All of the core game’s “Edit in CAS” basic functions have household size hard-coded to 8. So no modder can prevent those functions from deleting extra Sims. However, there is a different “CAS Edit” that allows editing those large household sizes without losing extra family members. You would use that CAS edit rather than “Manage Household” or a dresser or a closet or a mirror or anything else. The safest thing to do is just use MCCC to edit the Sim rather than a Sims 4 method. The option for editing a single Sim in MCCC is "Modify in CAS". It is available in the initial MCCC Sims menu when clicking on a Sim in a household that has more than 8 Sims in it. It can also be found in the “MCCC > Sim Commands > Modify in CAS” path for any Sim.