I play only one save, the one that was started back in 2014. I'm amazed that it still works. And - without any way I can stop it, the population is growing - and that is *before* we get infants... Last time I looked there was nearly 600 sims. I think there might be 120 households, all actively played, but yup, some visited just quickly to adjust with careers and births etc.
I focus a lot on community building, and the more I add the better I understand how things should have been done differently in the past. So, there is always development. I have a kingdom ruled through 3 chambers, where of one has 21 politicians including a President and a VP. There are 5 years elections and currently 3 political parties. There is also nobility with a good range of dukes, counts and barons. When I realized my royal family had only lower ranked sims to mingle, I founded 2 more kingdoms, LOL, but those are mostly the royal family and hardly any stately system.
I like to make essential stuff available to the common sims, ie I use cc corkboards with stuff to hang on it, and I create new swatches to allow sims to decorate with anything from favorite party logo to signed card from their favorite actress or musician. Elders like to add a photo of the royal family, LOL. I also make CC to have tons of nice and formal family portraits on the manor's walls. The royal palace has tons, but other finer Houses often have a handful, too.
And I keep family trees and profile pages for all sims, many trees going back 6-7 generations. I actually played Sim Medieval just to learn a bit about their founders and how they looked :)
Half the fun for me is to plan and organize, and most of it is available on my
TS4 site (tons more contents than you will ever want to see, LOL), plus
my blog. I also have a local Newspaper online.