If I don't have any specific story for the game play I usually always start with a single Sim. It can be female or male, whatever I feel like at the moment. I often randomize the traits and aspiration and start playing. I will decide later if and when my Sim will have a family etc. It depends on my Sim's personality and whatever happens in the game.
If I have a story in my mind (or if play a challenge) I create the Sims that I need for it. Sometimes it's a family, sometimes roommates, sometimes single Sim, whatever is required.
