I usually create one sim and go from there. I get bored easily though.
I once created 7 single sims (each one to represent one of the 7 sins) and played them on rotation. That was good fun. I still have that save but not played it in forever.
I also have a save that I am slowly rebuilding every neighbourhood.
My current main save is a super sim challenge. I realised my super sim would need to start as a baby to gain all the toddler bonuses so I created a single female sim with the intention of maxing out cooking, gardening, handiness and parenting skills, then having them become pregnant by a spellcaster and the child will then become my super sim.
I am having a lot of fun playing with the "mother" so have turned aging off for now. I started the game as her being a homeless teen with no money. They are now a YA studying an advanced degree in biology, have their own house, a garden full of magnificent harvestables, a cow plant called Moo and an adopted dog called Willow. They have maxed out 5 skills but are only at Level 1 in parenting (oops). Once they become an adult I will turn my focus back to creating a super sim child