Want a quick litmus test for the health of your game? Ask one of your players how many NPCs they can name. If that number can be counted on one hand, that's a red flag.
This may seem counterintuitive at first - after all, games are about the characters, and we all know the dangers of the GM falling in love with her NPCs - but it's never quite so simple as that. NPCs are a necessary part of the landscape for a healthy game for a number of reasons.
First, and perhaps most simply, if you only have one or two NPCs, then they're more likely to be the worst kind of NPCs: Elminster style blunt instruments used to beat the players down the path chosen by the GM.
Beyond that though, NPCs are important because they are the anchor points for motivations. Consider almost any motivation powerful enough to drive a character in play, and try to imagine how that works without other characters. Even seemingly internal goals, like growing stronger, need people to be tested against. Enemies provide competition and anger. Allies provide opportunity to prove yourself and sympathy. More complicated relationships spawn more complicated inspiration.
NPCs also provide handles for players to grab onto when they are looking for direction. It's not uncommon for players to find themselves at loose ends, either between adventures or at a point of frustration, and having NPCs on top of mind give an easy way to address that. Enemies can be pursued, allies can be consulted - for players, a known NPC is like a door in a dungeon room. They can open it at their leisure.
They also provide a point of comparison. NPCs can give a sense of how the world works, and give the players a sense of how they're doing. Fighting someone once doesn't tell you much, but fight them twice, and you have a story. Admittedly, this is a dangerous point, since this element of NPCs also contains the "Drizzt will always kick your ass" school of thought, but it's an unfortunate possibility, not a necessity.
So, here's the thing. I just spend some number of words defending the necessity of NPCs, which seems like it should be utterly necessary. Every GM knows this, after all - NPCs are one of the key building blocks of the world. Your game is, I do not doubt, utterly teaming with NPCs. You could probably name a dozen without even checking your notes.
But that's why the litmus test isn't about you. The number of NPCs you _have_ in the game is almost irrelevant. What matters is how many NPCs in your game have registered on your player's radar as anything more than "That guy from that one thing that time." No matter how crystal clear your NPCs are to you, if they're not in your players' minds, they're not helping the game.
[back] 1 - I'm leaving out the very important question of how many of these roles can be filled by other PCs for the simple reason that it's a bit of a doozy. Short answer, yes, other PCs can fill a lot of these roles, but it creates a very different feel for play. Whether that's a good or bad thing is uterly a function of taste.
[back] 2 - This is, of course, also true of almost every other plot element. I've known far too many GMs to gnash their teeth at their players 'not wanting to role-play' because they don't realize that they haven't actually provided anything to role-play *with*.