I have a few little side saves (like my save for JTB, which I didn't really want in my main save), but I mostly play one large rotational save that has been ongoing since I started the game. I, too, play in a loose rotation; some households I play more frequently or for longer periods; sometimes it depends on my mood, sometimes it depends on what new gameplay I want to check out.
I couldn't tell you exactly how many households I have, but I'm usually hitting right around, or just under 200 played Sims. I try to keep 200 as the cap for performance, but there are some "background" sims that I move in and out of unplayed periodically, whenever I need room to have a baby, add a pet, etc. I've never had a problem with any of those being culled, as they are housed, have jobs, and have relationships with my played sims. (I also regularly cull the unplayed sims that are automatically generated.)
Someone pointed out to me a few years ago that the premades/townies for various worlds often have a hidden trait to indicate that they are a native of that world. (E.g., Sulani native, Henford native, etc.). So when I start a new pack, I go in and evict all of the premades, then add them to one household with maybe some townies, local sims from the gallery, or sims of my own creation, and I make sure that they already have (or I add) that hidden trait, before evicting them all again. That way, I can free up houses, while also improving my odds of having locals of my own choice wandering around, instead of just, like, Nancy and Katrina. However, sometimes I will also temporarily leave the premades in their original homes until I actually have a need to move in a different household.
While I don't play with most of the premades, I do at least occasionally check in on the classics, like the Goths and Landgraabs, and definitely give them jobs, update their houses, maybe add family or other relationships. For TS4, I had to add a Worthington family (frenemies of the Landgraabs), because I was surprised they haven't shown up yet, since they date back to at least TS2 in both lore and premades.
Usually I just wait for ideas for new households to come to me, based on new pack content, a name I heard that I thought was cool, old sims lore, etc. I'm not actually planning to create any new households for Growing Together, but I'm going to start creating families with several of my existing households that have been on hold for years. Oh, and not surprisingly, I play with aging off. Though with the new pack, I expect I will soon start using manual age-ups more frequently than I have so far.