This is a curious topic for me as I am a monster-junkie but how many different monsters does a campaign need? Not the game itself the game could have thousands of different types of wicked humanoids, devilish fiends from beyond, and gobbling grues in volume after volume but ... how many does a campaign actually need?
Years back I made a gentleman's bet with another D&D player that you could run a healthy campaign with 60 or so monsters, I did weasel out and have normal animals and player races excluded from the count of monsters. I dug into my monster books of the time and came up with a trimmed roster of monsters that fit the campaign and they did the trick for a few years until the campaign grew beyond it's original scope.
About 60 monsters did the job fine and dandy when the campaign focused on a large valley region of a continent. Some adventures could start out elsewhere onto the continent but most took place in the same 5,120 square mile region. I do not have the original monster roster on hand any longer but it did include an abbreviated humanoid line-up of Kobold, Goblin, Orc, Gnoll, and Bugbear. There was a short list of giant-kind including Ogre, Troll, Stone Giant, and Fire Giant. Undead had the classical mix for Skeleton, Zombie, Ghoul, Wight, Wraith, Spectre, and Vampire with Death Knights topping out the list. I Used the chromatic dragons and just Silver and Gold for the metallic ones. There were a few fairy types and a number of odd-ball monsters (mostly from the fiend Folio). When you got down to it about 60 types of monsters, player races (as NPCS), and normal animals was probably crowding up the place.
No one noticed a shortage of monsters and it wasn't until the campaign went deep into the underdark and to foreign continents, other worlds, and times before I had to expand the roster of what was likely to be creeping behind a door and down a tunnel. It's clear from that (for me) that a tailored monster list helps define a campaign setting as I didn't feel the need fro more until the players and I stretched the size and scope of the campaign.
So I suppose I knew the answer the whole time before I asked the question: A campaign needs as many monsters as it takes to define the campaign and entertain the players. You want to meet some expectations but you also want to change things up a bit; while I restricted myself to a small variety of monsters (for post monster manual D&D) for a good long time I did make use of gimmicks like character levels for intelligent foes, unique equipment, and changing the relation ship of monsters. One area (composed of the orignal restricted roster) was plagued by berserker cultists with elite members able to change themselves into werewolves, all lead by bad-guy druids; that arrangement worked well enough the PCs relationship with druids was always a tad restrained and bersekers were feared much more than their 1+1 HD would indicate they should be. Trying to do more with less led to more inventive and creative use of monsters and reduced the monster zoo feeling of dungeons.
So how many monsters do you think a a campaign needs?