I have only one save that I have played about 3,5 years. I like to have all my sims there, so that they can socialise with each other and I like to see how their stories intertwine with each other. At first I just played many families, but now I have three main families that are like legacy families and then I have background sims whom I play if I want to.
I find having one save to be easier because I wouldn't have time or energy to work on many different saves. Sometimes I'd like to start a new save, but then I remind myself that I can change my current save and add the stuff I want there.