My big save is a rotational one that is up to 21 households now. I'll be adding three more in the next rotation, but I'm not quite halfway through the current one right now.
I play with aging on for the active household only, so any household with sims who age has to be played once per rotation to keep them aligned. I don't play the households in the same order each time. I have a spreadsheet where I track which holidays, festivals and phases of the moon are in which week, and plot out my order based on who I want to play doing what. It usually changes a few times. I recently bumped up the retirement community because I wanted to move them to San Sequoia after I bought the new pack, so they swapped places with the mermaids. Every household gets one week per rotation, Tues-Mon. I have 4 week seasons and lots of holidays, with 16 day moon cycles.
I started my save with one sim, Athena Andreas, who had nine children. Her descendants I play like a legacy, but all the spares found new households. For the most part they marry in premade sims, but I have a few exceptions. Gen 4 is still in development, with the oldest members just aging up to teen and many more yet to be born.
I identify my households alphabetically. Athena founded household A (Andreas), then her 8 spare children founded families B-I (Bateman, Caliente, Delgato, Eggleston, Fyres, Goth, Holiday, and Ibis), and her spare grandchildren are in the process of founding families J-N as they age up (Jang, Kalani, Le Chien, Munch and Ngata). I'm saving O&P for the great grandchildren (Prescott, but no idea on O for now). Some of the spares have started moving into my occult households instead of founding new families, to slow the expansion. Every branch of the family is assigned to 1 or 2 packs to help me in my goal to use all of the content at least once in this save, and they are also part of my goal to finish every career once with and once without a degree, and finish every aspiration.
Then I have a household for every occult from R-Y (robot, spellcaster, Tartosa's occult mix, undead/ghosts, vampires, werewolves, xtraterrestrials, and y is the mermaid tail). I'm holding Q for a future occult even though there's no chance the letter will fit as nicely as the others. Some of the occults are helping out with the career completionist goal, mainly the ones who grew up in the families. But mostly these households are focused on the occult skill trees, and most of them have a story focus rather than a goal-oriented one.
Then I have life stage households, which right now just means the retirement village as household Z. I am going to make a high school dorm for students from Sulani and Mt Komorebi and other rich students, and then I'm going to have university student housing eventually. But the 4th generation will be the first to go to active high school and to university so those don't exist yet. There aren't any letters left so I haven't decided what to call them.
I've been playing this save for 4 years irl, though these last 2 I've put in a lot fewer hours than the first 2.