I only have two. One is the one I started with in 2019 when I started playing Sims 4 and the other is the one I actually play with.
Every once in awhile I load up the first one to edit, save, and make a new copy of. Usually when there's a new world. Then I use that copy as my new play save. It's slowly getting filled with all my own builds or builds from the gallery and all the premades have been replaced.
I have one Sim that I've had since 2019. I save updated copies of her (and her house) to my library, then put her into the new save each time I start a new one. She has had kids, grandkids, great grandkids, great great grandkids... But with each new save, she's alone because I don't also save the family. I did at least get to 11 generations once before I started a new save. I just eventually get bored and want to start fresh with her.
But I am very attached to her at this point. I've never had one Sim for so long. She's a vampire, and I have neighborhood stories turned off for her, so I don't have to worry about her dying when I'm playing with her descendants.