There is an excellent post over at Aeons & Augauries about the number of monsters in a campaign.
The short version is that he did a campaign with about 60 monsters, excluding normal animals and PC races.
My current count is 82 discrete monsters, with a lot of sub-types counted as one (all the various zombie types are just zombies, for example.) That includes normal animals, though - if they were a combat encounter, they are listed. If they aren't identified, just seen, or act as wandering damage (like my bug swarms in the Cold Fens), they don't make the list.
Monsters I've Used So Far
But even so, there are piles of monsters on my list that are already stocked into the Cold Fens and the depths of Felltower that haven't been encountered yet. I think my game will end up with more like 120-150 monsters encountered.
Part of that is the genre. It's Dungeon Fantasy. It's a bash-the-monsters game. It's a loot-the-trapped-chests game. You know there is going to be a lot of monster fighting. It's a game that thrives on a variety of monsters and variations off of that variety.
Part of it is because it's my goal. I'm going to use every stupid monster mini I own. Every cool one. Every weird one. Every oddball ones I ended up with for no reason I can explain. And I'm using some things that aren't even intended to be minis as minis because, you know, I can.
Not only that, but I hope to use every single monster from DF2 and from DFM1. Every. Single. One. The ones I made up I've mostly used before or used already, or have a place for in my games. The ones Sean made up I just want to use because Sean made cool monsters.
Like I said, though, it's that kind of game.
I used a much tinier subset of monsters in my previous GURPS game that went on for 10 years. And, as you can see in the post, the idea of a thinned monster list can make for a real change of pace when the venue changes. If you've used a subset in a specific area of the campaign world, it feels like a real change when you move to a new area and suddenly the monsters change. If every badlands in the whole world is full of orcs, gnolls, and bugbears, it doesn't matter which badlands you are in. If the Eastern Badlands are full of gnolls and the Southern Badlands are full of Bugbears, which the PCs haven't seen to date, a change of locale really does make an impact.
Not only that, but using a subset allows the players to get a feel for what's out there, predict based on a smaller set of possibilities, and general get comfortable with the game world. As much as I'm throwing the entire collection at my players this game, I recognize that what you exclude defines your game and shapes it as much as what you include.
Really thought provoking post. If you've gotten this far in mine without reading it, please go and do so.
And count the monsters you've used. It's a lot of fun.