You can then switch back and forth between the multiple households (this is called 'rotational playing,' but you don't have to play them all the same amount of time unless you feel like it). You can also create a new played household in an existing file, by leaving the family you're playing now and playing either one of the prebuilt households or a new household that you create. That said, it's a reset and Strangerville will be strange again, Sulani will have trash again, your neighborhood eco footprints will go back to default, any buildings you added to your other save won't be there. This is 'starting a new game' or 'starting a new save.' What you do in this save doesn't affect anything in any other saves, which makes it great for experimenting, or just because you have a cool new idea or challenge you want to play. If you pick 'start a new game' or whatever, everything is set back to the way it was when the game starts (including Strangerville) and you can create new households and move into new homes and start that new Sim struggle all over again.
I probably have 20, although I only regularly play with 4 or so.