There's an option to limit the number of sims protected before culling happens in Options. This number also shows on the Household Manager UI at the bottom. This doesn't stop more being generated if your system can handle it, though. It does limit the number you can protect by adding them to My Households.
Unfortunately it jumps from 200 (capped) to unlimited, and if you play with premade sims then there are already more than 200 across all worlds, so the setting has to be used on unlimited to keep them all safe.
With MCCC, not only can you limit the number of extra sims generated, but you can also limit how they spawn: You can draw sims only from your tray, limit sliders so you don't get fat/thin/muscley sims, limit outfits by only having townies spawn in pre-saved outfits, clean townie outfits by banning specific items etc.
