How many, indeed? For every ninja you count, you know there will be others… hidden, ready to strike at any m- ARGH!
How many actors?
You’ve never smashed a clock?
Enough to spontaneously start re-enacting Battlefield Earth in complete earnestness.
Coconut crabs.
Three. Enough so that I think I’d have a proper count/location on all of them, but also enough to hint that I might be missing one of them…
Live squid. (The invertebrates, not the Navy type)
Small ones, or giant ones?
A swarm of small ones, I guess, but one giant one would freak me out!
Cockroaches.
Two, because then they might reproduce, and then they’d be everywhere!
Cats.
Do the cats already know each other or not? If they do, and are reasonably friendly, I could take a dozen or more – but if I were locked in a room with two hostile cats – shudder
Koala bears
12, they’re cute, but fragile little buggers.
Giant earthworms, eh? If I’m off for a weekend fishing excursion, then I’ll need two or three. Any more would be a little frightening.
Ok, hmm… how many SDMB moderators?
No limit. They’re all extremely nice and lovely persons who I’d love to hang out with.
(is my tongue brown enough?)
How many hamsters?
I’d be willing to put up with a billion if the Boards would run faster.
Misfit toys.
Koalas? Fragile? I have seen two koalas fight and let me tell you, they are far from fragile. They may look sleepy a lot of the time but they can really move when they have to.
Twenty would be scary. Fewer would be odd, but twenty and they’re surrounding you and in your peripheral vision and getting behind you…(shudder)
Spider monkeys.
Spider monkeys - if they’re repeating everything I say, then just one is annoying, but three is spooky.
How many America’s Next Top Models?
Two. Doesn’t have to be the American version tho, they can be from any country.
How many lantern collectors?
One. The freaks. 
How many professional you look-alikes.
Don’t take this the wrong way, twickster, but when I started reading your post and saw that you were looking for your first post, I thought that you were going to ask, “How many posts do you come up with when you search for your own posts, and how scary is that?” I mean, I registered before you and you have some 21,000 plus more posts than me. That’s a lotta postin’!
But on the topic:
My initial thought is “one clown is one clown too many.” But I knew someone would say something like that, so my second choice is, “anything more than three cats that are all in the same room, looking at you and slowly creeping towards you silently.”
I’m just going to ignore Cuckoorex, who obviously has a life.
I think one would do it – esp. if she’d mastered my annoying mannerisms. If there were any, I mean.
hears a bark from next door
Corgis
π corgis.
Heck, one is bad enough. An irrational number of small yippy things would drive me to gibbering fits.
Goldfish
Lying dying on the floor? Then two would be enough to make me sad 
Investment Bankers?
I dated one for a while – must say, I don’t have a problem with the idea of him lying dying on the floor. Self-centered jerk grumble grumble grumble
One.
Praying mantises