Click on any sim's computer. Click on MC Command Center/ MCCC Settings / Gameplay Settings / Maximum Household Size
The default in MCCC is the game's default of 8. You can change that to anything under 25.
To edit a household and have it keep more than 8, you have to click on a sim who is in the household you want to add more to. Click on MC Command Center / Modify Household in CAS.
You HAVE to use the "Modify Household in CAS" method to add more than 8 or the game won't keep it.
FYI... Setting the time speed to 25 is only necessary if you also have Simulation Lag Fix. SLF doesn't work well at all if you have it set to other than 25. If you don't have SLF then 25 can be changed just fine. If you don't care about longer days then just leave it.
