I left it between 6-10 families per save.
I tend to rotate between families, and leave the aging off, unless I have multiple families and expecting babies, I will put aging on. With the babies I just change their names, and fit them how I want, and then turn aging off again.
The reason I have so many families is to keep my interest. Not every family of mine follows the traditional family, and it allows me to do different things. Plus, I like seeing my families interact with each other when I am not playing them at the moment. Also, having families makes my worlds feel more lived in.
I have at least 5-6 save files, two of them are currently dedicated to series I like, which I am starting a Harry Potter one and already have a Frozen one. Those save files have canon, semi-canon, AU, and a few genderbends all in the same world. They also have at least one family that isn't related to the series.
I have two save files that are a mixture of Sims I create that aren't related to anything, and others I download or create that are based on a series like Lord of the Rings. So, normal Sims interacting with fandom Sims.
My final save file is dedicated to families I created in Sims 2, and making a version of them in the Sims 4.
Also, making different families is really fun, because I like being in the same world, but want to play a different family that day.