There were 5 Monster Manuals for D&D 3E. We're up to 5 Bestiary books for Pathfinder now. That's not counting various smaller monster books for the Forgotten Realms and the Inner Sea of Golarion. Original AD&D really only had 3 over the course of 10 years and we somehow survived and I don't think the world's monster consumption numbers are up that much over the 80's. I've never sat down to put together something for a game and said "wow, I really need more monsters", even in games where there was only (gasp) a single book of monsters available. This is especially true of games like PF and 3E where you can stick class levels on monsters, give them feats and skills, and give them magic items, resulting in near-infinite possibilities. There are "templates" in those games as well which just open up things even more. I've never understood the idea of someone being "tired" of the monsters available in a game. Tired of what, flipping past them in the book? Because I've run campaigns for years and never used up all of the monsters in any Monster Manual type book.
I suppose it's partially a production problem - a company needs to sell books, monster books are easy to put together, and they seem to sell well enough. Crank one out every year or two and there's a slot on the schedule filled. The thing is I am sure we are well past 1000 different monster entries in Pathfinder alone just in the 4 Bestiaries I own and I will likely never use even half of them - so why do I need the 5th? Or the 6th?
Superhero games do this too. I will make an allowance that super-games tend to feature unique opponents so there is more value to having more completely different enemy entries than you might have in a fantasy game. Even considering that though, supers games tend to accumulate a ridiculous number of villain books in a short period of time. Champions in the 90's had Classic Enemies, Hi-Tech Enemies, European Enemies, Alien Enemies, The Mutant File, and then Allies (or enemies for your enemies) and probably more that am forgetting because I am not looking at the Champions shelf right now. Icons has multiple enemy books out now and M&M does too. I will never use even half of them in any of these games, so it's a case of diminishing returns.
That's an important thing to keep in mind - the law of diminishing returns is huge here. Let's call it "Diminishing Utility" instead in this case.
- The first monster book or enemy manual in most games is crucial as it sets the baseline for mechanics and for presentation and implies some things about the setting too. A lot of times this is integrated into the main rulebook (from basic D&D to FASA Star Trek to Dungeon Crawl Classics).
- The second may be useful for covering any legacy monsters left out of the first book or for expanding beyond the legacy stuff that was included in that first book to show what else can be done.
- After that I personally find the utility drops off tremendously. At this point themed books can be useful: Bestiary 4 for PF focused on Mythic Monsters, which is very useful when running a campaign that uses the Mythic rules, which I am doing, but another "general book of 300 bad guys" is not that exciting.
|Then there are the third party monster books - some are good, some are bad, few are essential|
How does the game keep growing in the monster department in a healthy, hi-quality way? Put the opposition in your adventures! Early D&D did this - the Drow first appeared in the back of a module! The Drow! D&D's "Borg"! (In the sense of being an interesting and terrifying opposing faction added after the basic structure of the universe had already been established). This is pretty standard with superhero adventures - gotta have a villain! - and is one good reason to pick them up! It also means your new enemies have context far beyond a stat block in a huge book of stat blocks and become a far more personal opponent. It's the difference between introducing a one-off alien in a Star Trek episode and introducing the Klingons. If your new enemy/monster/supervillain is actually that interesting then they should be able to support an adventure on their own. If they cannot, they're probably not that interesting and if that's the case why bother adding them to the game? Keep working and come up with something better! once you have a certain quantity of opposition, it's time to work on quality of opposition. The game will be better for it.