I have multiple save files, and different things going on, as well as similar things.
For my PC version, I have at least 8-10 save files, but each one just has one family. Those save files I plan for the one family to extend through the save file through generations.
My PS 4 version is a whole other animal. I have at least 6-7 at the moment, and the best way to describe it is insane. My biggest save file also serves as my test file. In it I was on a Frozen kick, so my Sims are Anna, Elsa, and Kristoff based on canon, AU, and other stuff all living together. I threw in some other Sims too to break up having just Sims based on Frozen. It is hilarious seeing my magic realm made up of nothing but Elsa Sims, though. I also plan to change the file through generations, for example, one family has a family I named one kid Hiccup, and another one Astrid, so I plan to change it up through generations.
Two other big save files I have are similar, I have normal Sims in both, but each one is filled up with Sims based on series I like. I like a series called Natsume's book of friends, and have my own version of the main character, as well as ones downloaded from the Gallery. I have more of a mixture of series I like in another save file.
One save file I am working on is loosely based off of Harry Potter. I have five families so far, but plan to add more. My main three are Ron, Harry, and Hermione as toddlers in their families. I am keeping Harry's parents alive, and have to leave the Ginny as an in-game born Sim, so there may end up being more Weasleys. My other two families are Harry and Ginny with their kids as teens, and Ron and Hermione with their kids as teens. Still working on the save file on and off.
I have another save file with three families so far. Two were a separate Elsa and Anna, who I had pursuing careers. I wanted to focus on careers, so they had one to themselves. I gave Elsa two teens, and Anna with two teens, and a child she had with Kristoff. I gave Elsa a boyfriend when I moved her out, and edited a kid for her. She and her boyfriend don't like each other, but love their daughter. They were put on the back burner, because I used the save file for a scenario, and the Sim I made for ended up getting with Clement Frost. So, once I am done with the scenario, those two will be the founders on an not so planned legacy.
I have another save file that I moved large families I created in CAS, and made them all neighbors. I wanted to experiment with looks, and wanted a place to put them, so I made a save file for them.
My last current save file is based on my Sims 2 game. I used to keep a list of the Sims I created for Sims 2, and made them again for Sims 4. I have most of the families I created for Sims 2 in there, and may add a bit more, or keep the population smaller for now.
I still plan to create some save files based on Avatar the last Airbender and Legend of Korra. As well as some other series I like. For my Avatar and Korra it may be generational like the series, or have Zuko and Mako the same age. It may be a mix of both. For series I like, I tend to create a save file dedicated just to them, however other series I like may be mixed into save files with normal Sims. I also don't want to fill up the entire save file since there isn't much room for the amount of Sims I want to include. In my first save file, almost every single available house is filled as well as houses I pulled from the Gallery. So, I have to plan my goals and save files going forward.