Twojeffs at simbology has it I believe - just google it and you should find it no problems. In fact, hang on a minute and I'll take a look...
*edit* Here you go!
http://www.simbology.com/smf/index.php/topic,18.0.html"Adds Triplets and Quads to the possible birth options, with ability to choose how many babies you want a sim to have. Option to randomly get 1, 2, 3, or 4 babies, plus ability to set your own odds."
I've not seen any that go any higher than 4. Of course, you could just get the sim pregnant again straight away and speed up the pregnancy.
HTH!
Hannah xx