I definitely think that 100 is WAY lower than the actual number for most RPGs.

However, you only have a few basic forms of RPGs and their NPC numbers vary greatly.

1) Console: These games tend to be similar to FF6 and earlier. When the games first came out, you'd have 12-20 NPCs per 'town', with a few that weren't related to any town. With 4-10 towns, you usually ended up with between 75 and 150 NPCs that you could interact with in various ways. Some were vendors, some were 'quest' NPCs, some were background NPCs, etc... Early PC RPGs tend to be similar in their scope. With the advent of CD and DVD-based game systems, these games are going by the wayside, making way for the next category of RPG.

2) Epic RPGs: Morrowind, FF7, MMORPGs, Baldur's Gate, etc... These games are built around not only a huge scope of story, but a huge world where your players can take a lot more paths to the same end. They commonly *start* with hundreds of NPCs and grow from there, some (Morrowind) potentially hitting 1,000 or more NPCs. The vast majority of these NPCs are quest-related in some fashion.

3) Hybrid RPGs: Diablo type and RPG/RTS hybrids. These games actually harken to the classic Console genre of RPGs. Most NPCs are there just to give the world a little (and I mean *little*) depth and mostly the vendor/trainer type. They usually start at 20 and go up to 100 NPCs for the larger ones.

What style that you pursue really matters on your view of the game. If you want something that is Diablo-esque, you can get away with a relatively small number of NPCs. if you want something that is more along the lines of Morrowind, then you're going to spend a lot of time building NPCs. Fortunately once you get above a certain number, they get relatively cookie-cutter. Most companies make a few templates for NPCs and simply insert the appropriate type where they want it. If they're smart, they allow for hybrid NPCs (vendor/quest or quest/storyline NPCs for example), so that you're not stuck with a world that is *too* bland.

NPCs (count) isn't usually where you run into problems, you'll probably run into more problems with storyline and consistency than you could with your NPCs. Creating somewhat useless NPCs might flesh out the world, but really do no more than waste your team's resources, which could be better utilized adding in new side quests or features. The #1 rule of thumb for NPCs nowadays is: NPCs without a purpose are wasted pixels. The systems of today could easily handle the AI to deal with extraneous NPCs, but there is no reason for them to. You'll just waste the player's time running around trying to find the NPCs that are actually figure out whether each NPC is worth visiting.