[Tyhm]

Not so many NPCs. You could do it with one very-complex NPC per player, or many simple NPCs per world; either each player's responsible for all NPCs under their control, or all NPCs except for the active ones use fairly simplistic idle scripts.
To do a Starcraft type server, you'd probably need to do something along the lines of every player controlling their entire army in a single NPC. For a visual aid, figure the actual NPC(w) has a gameboy icon and while it's active, it draws all these troops. Every time your troops open fire, it tells any gameboys it's linked to "We're shooting at I-5", and the other gameboys say "You sunk my battleship!" and draw the corresponding loss of their units.
Not too terribly complex even, particularly if they aren't allowed to fire blindly and ask "Anyone in I-5 now? How about now? How about now? Now? How about now?"