So I've used like 108 identified monsters in my game, plus some not yet identified.
Why so many?
Short version is I like monsters and the more the merrier. There are counter-arguments to using a lot of monsters. I've thought about them, and here is why I didn't go with them.
Using Too Many Monsters Diffuses Their Value
Pick a subset of monsters and stick with them. It gives you campaign a tighter theme, it makes the players value knowledge of monsters more, it gives your campaign a flavor that isn't just "everything in all of those Monster Manuals."
I agree with those statements.
But I rejected that idea for my campaign this time around. The is a game predicated on new, weird, dangerous challenges. A wide net cast across my minis collection, over a dozen monster books (at least), many games, all the Cardboard Heroes I've collected, things my players have mentioned dreading or liking, etc. I want a chance to use them all. All of them is the theme. All of them pulls it together. All of them makes the players work that much harder to know what the heck they're facing. And "surprise, it breathes acid and is immune to cutting weapons!" is the kind of fun this game is meant to have. I could do that with fewer monsters, but again, the goal is "as many as possible."
Unique monsters are scarier
Probably true. But it's more work. I need to make a lot of unique monsters that aren't weird for the sake of weird, something I disliked in Isle of the Unknown. Also, unique monsters are much cooler when there is buildup. You meet the Black Beast of Aaarrr, you all know about the dreaded Hydra, a quest to deal with Medusa means you have to get special help because you know how bad it'll be.
In a dungeon crawl, that's a lot of buildup. I think that fits mythic heroism and horror much better. Dracula is only defeatable if you know his weaknesses, the Hydra only slain(ish) when you know what to do about its stumps . . . and you aren't tough enough to beat it to a pulp and then guess until you get it right. In other games, knowing is half the battle. In my DF game, it's more 0-100%, depending on the monster.
Man is the Real Monster
I'm pretty sure I live in that world. So I don't need to have a game featuring literal monsters if I'm going to undercut it by having them be second fiddle at the only thing they do. You know, monst. I have really horrible humans in my game, but yeah, I also have monsters of pure evil.
So yeah, man is the real monster, unique is scarier, and fewer monsters is a tighter theme. But that's not the game I'm running.