As a player I usually focus on one Main Character, and a second I could sub if my main needed a break, died, or was taken out of action a while. I do this mostly because my mind tends to follow possibilities quickly with this sort of thing. Setting out to make one character is a fool's errand for me, as I already know one or two more will sprout up. Anything more is just thrown into the NPC list and I am happy to see others use them for their plots. 

When GMing I try to form a balance. In the beginning, a GM is a bit like an author in that they must put enough down for others to get the jist of the world and it's possibilities. I make a few NPCs or suggest their existence, then allow my players to take over, build those personalities, and ideally build up the world. In my eyes if I'm the only one creating the population then I may as well go write a book. So here I focus on one player and urge everyone into creating NPCs to liven the world.