Note - alters a game critical file. Back up the file elsewhere, or you screw yourself with a neat re-install or try to track down and kill the errant file.

Locate the SOO29582617-config-log file in the game install directory (i.e., My Documents/EA Games/The Sims 2/Logs). Open the file with Notepad.exe or similar text editor and locate the string: uintProp maxTotalPets 6

During the game you would bring up the cheat window using CTRL + SHFT + C and type in the code replacing the 6 with a higher number such as 25, you would be able to have more pets in your house.

However, the only way to get more pets would be to create them in the create-a-family mode, and then combine those pets and your household together. However, once you have 6 pets, none of your pets can become pregnant (even with the cheat). Also, you cannot adopt another pet, marry someone who has pets, or have someone move in with more pets, even with the cheat activated. But hey, at least there's one way you can get more pets.

Also, next to that line of code in the text document were the codes: uintProp maxTotalHumans 8 OR uintProp maxTotalSims 10

I tried those, and I suppose they have something to do with how many sims can live on your lot, but I'm really not yet sure how they work. Maybe someone else can make more sense of it.