Relatively frequently for a variety of reasons.
I'll add in a NPC or two if there's only a couple of players.
I'll sometimes add a "plot" NPC to the group.
If the players are interested, I'll sometimes give them some potential henchmen to recruit (ala 1E and BECMI).
And, despite the supposed taboo, I'll occasionally toss in a DMPC if there's a concept I really want to experiment with but will never get the chance to actually play.
In all cases, the NPCs defer to the PCs in decision-making and such.