Adding sims to a household isn't an issue regarding your game, it's an issue regarding your rig. On average, the standard set up for a PC
(2GB of RAM, Graphics card with Pixel Shader etc) can handle a decent amount of on screen simmage. But if you're like me, on an older rig (My PC is at least 8 years old and only runs of 512MB of RAM) having more then 10 sims on a lot is a bain to one psyche. So before you go looking for codes, mods or anything that will help add to your ability to add more sims to a lot, check your hardware and see if it's up to snuff.
Now, let's say you're hardware is good enough to handle the work load. Then the problem is simple, you need to get more juice out of your set up. How do you do that???
http://www.insimenator.org/index.php/topic,105572.0.html <<<-CLICK ME!!!
My tutorial on how to increase virtual memory (VM) for Windows XP. At the bottom of the post there's also a link on how to do so on Vista. Follow the steps posted within to pump out more power and improve your PC's performance. That should in most cases alleviate the issue of not being able to have enough sims on a lot.
BUT... If you've done both previously mentioned tasks, and it's STILL not letting you add sims to your existing, jam packed families...Here's what you're looking for...
http://www.modthesims.info/download.php?t=250640 (You have to join MTS in order to download...)
This bad boy allows you to add
50 FAMILY MEMBERS (30 sims and 20 pets) to a household. PLEASE read the description on the mod in full before putting it into your DL folder. All risk in adding this mod to your game are solely on you to take, so make sure to find out how to work it, where to install it, and it's limitations, conflicts and so on by reading the description. Also...to be safe...go ahead and increase your VM with my tutorial regardless, because if you're gonna be adding anything more then 10 sims to family, your rigs gonna need some serious performance enhancing. (It'll be just like a MLB player.

SHWEET!!!)
Good luck, and happy simming.