One of the FAQ’s & concerns around the Sims community is the household limit with Pets. Today we have our answer confirmed via 
Gamescom 2011.

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The Sims 3 Pets Expansion has bumped the household limit by 2 totaling 10 members. This means you can mix and match between 8 Sims and 6 Pets that do not exceed 10. Pets will count as member of the household and take up a spot.

Note that only Playable Pets (Dogs, Horses & Cats) take up a spot in a household panel. Lizards, Birds and other collectible pets can be acquired by the dozens. (assuming you have enough money)