Assuming the earth is flat, how many people are part of the conspiracy

Add the entire population of Denmark. Country is dead flat yet you can't see one city from another one. Nor can you see the mountains of neighboring countries.
- The Netherlands enters the chat.
  
  Cue the Dutch that live 50m NAP. Sometimes it doesn’t even feel like you are a part of the same country.. 🥲

The guards that patrol the ice wall.
Entirety of NASA. Entirety of NOAA. Meteorologists. Cartographers. Everyone who works on Google Earth. Every engineer who works on satellites, rockets, and planes. Physicists.
However, I do think 10% is probably too high an estimate. While these are a lot of people in a lot of areas, they represent pretty small demographics each.
- Wouldn't you assume that their family would know and be in on it? I was gonna suggest it has to be more than 10 percent. Since it's not a life or death conspiracy like the Manhattan project I would assume a lot of those people 'in on it' would tell spouses, best friends or even older kids. If you knew a huge secret like that would you want your close loved ones to be lied to and indoctrinated into a false truth?
  
  If you're in on the biggest conspiracy in the history of the world, you don't tell anyone about it. There are CIA agents who live their entire lives without their spouses knowing they work for the CIA. How much more powerful must the round earth agency be than the CIA to maintain such a conspiracy for thousands of years? I don't want to say illuminati, but it sounds an awful lot like the illuminati.

More than just people. Some form of greater intelligence has to be at work since anyone can run a simple experiment to prove it’s round.
- The ending of the Netflix documentary where the flat earthers prove the earth is round is hysterical.
  
  What's the documentary called?
  
  Is that the one where they use the lighted poles at night and a board with a hole in it to prove the earth is flat? In that one they seemed a bit baffled and said they'd need to do the experiment again. To their credit, when they ran it again with the same results, they said they'd need to reevaluate their beliefs.
- That is unironically part of it. When people do experiments that show the earth is round they get told to pray and do it again. This suggests that their rationale is that the devil is tricking you into seeing bad results and if you pray hard enough you will eventually see five lights.
  
  Awesome reference.
- Even math is involved? Dang this thing goes further than I thought.

Anyone who lives near the sea an can see ships disappear behind the horizon.

This is a bit extreme, but maintaining a conspiracy with lots of "insiders" isn't impossible. Just think of how many people worked on the Manhattan project.
First you compartmentalize, so that only a core few know the whole picture. Then you threaten everyone involved with serious jail time for informing. (This is why we know Bob Lazar is full of shit, where he telling the truth, he'd be in prison)
It's true that secrets can't be kept forever, but they can be kept for a while.
The flaw in flat earth is not the number of conspirators, but the purpose of the conspiracy. It all falls apart when we ask why the government wants to keep flat earth from the public.
- Manhattan leaked before they even started work.
  From memory, a sci-fi magazine seller figured it out, just based on address changes. He even figured out it was likely an attempt to make an atomic bomb. The USSR also had it completely compromised. The first soviet bomb was basically a direct copy of the American one.
  That was also with most/all the workers in 1 location. They also had a very good reason to maintain security (grunts on the ground, not just leadership).
  
  Wasn't the Manhattan project compromised from within because of communist sympathizers, or even outright communists working on the project?
- To sell more globes, obviously.
  
  But if the flat earth goes on, what can be assumed to be, infinitely past the ice wall (since no one really gives an answer for what's past there), think of how big maps would be! They would have to sell giant maps and make a fortune.
- Flat earth is just a variation on the "It's the Jews!" bullshit that's existed forever. See Folding Ideas youtube video for details.
  
  When they gobble up one bullshit conspiracy, the next ones go down easy.
- They're protecting the turtles. If people like Bezos knew that it's turtles all the way down, they'd certainly harvest the turtle shells to make fancy hair combs. That would spell the end of the earth.
- the government can't afford to let the plebs know they live on a giant pizza. that would destabilize everything!

Literally everyone with eyes and a functioning brain who has ever charted the stars, flown in a plane, or looked out to sea. I'd wagers that's far more than 10%.

Since flat earth nonsense is just rebranded creationism, the only being who really needs to be part of the conspiracy is satan.
Then anyone who tells you that the earth is round is either deceived by satan, or is a satan worshiper.
This makes it super easy to dismiss anything that would hint at reality, like ships sailing over the horizon.

I was going to add other groups that would have to be in on it, but it is only required that you are all not real.
I am real. My family is real. But outside maybe 30 miles from my house, no one else needs to be real.
Sure, I can and have driven farther, but the trackers alert the actors. This has the advantage of the Earth's population can be just a few hundred thousand total.
Why? Clearly, I am the last truly important person, for reasons that are not yet clear.
So ... 50,000 people are in on it at least. And they are.... 40%+ of the Earth's population.
Edit: to finish the thought. OP's 10% puts those in on it at 800 million people. My 50 thousand is a much smaller number.
- Your family isn't real either. Neither is your house. This is all a product of your nascent consciousness.

GPS is in on the hoax, too. Think about how much they had to hide the fact that those satellites can't "orbit" a flat disc....
- I'm still trying to figure out why Russia used to own Alaska. Hell of a trip across the world to have sold that portion to the U.S.
  
  The way the disc works the center is the South Pole and the North Pole is actually a huge ice wall, it’s all very logical and easily validated by scientific evidence obviously
  Just the other day I was able to demonstrate how gravity works by spinning a disc covered in toothpaste very fast, all the toothpaste stayed on the disc

Don't forget everyone who works on the moon
- Checking in during moon lunch, thank you for making me feel seen.
  
  What time do you take moon lunch?
- And the ISS.
  
  No, that one is fake.
- Pssh. You still believe in the moon?
  
  Of course the moon is real, I visited in Minecraft!
  Mined some cheese on it as well.

The entirety of Australia, because Flat Earthers often deny that this continent even exists and claim that it's the "counterweight" of the plate or some sh*t like that, and everyone living there is a paid actor employed by NASA to cover up The Truth™
No idea what Flat Earthers living IN Australia think of this tho...
- Maybe yore thinking of the counterweight continent from Discworld?
  
  Where do you think Pratchett got the idea?! They got to him first and paid him off so we'd think it was ridiculous!
  No joke, it wasn't a flat earth thing, but I had a coworker years ago who was big into conspiracy theories and he claimed that movies like Men in Black were made to make everyone think that kind of thing only happened in fiction so we'd laugh at people who think it's real.
  When I tried to point out to him that there was no evidence for the things he claimed were real, he said the lack of evidence was proof, because it meant they were hiding the evidence.
- The thing about flat earthers is that they don't believe one thing, pretty much every group has a different believe system. The chance that a globe eartger and a flat eather have a similar system they believe in is higher than a flat earther and another random flat earther.

You're forgetting that with the microchips in vaccines, they can control our thoughts, so most of the participants don't need to be willing or aware. Cuts it down to just a few people in control of the 5G vaccine chip program.
You're welcome.
- And the "few people in control" are members of which out-group...? All aboard to Q-land, everyone. Final stop, domestic terrorism.

I've always wondered how the Soviet Union and the Cold War fit into the NASA conspiracy. Did Korolev receive paycheks from across the ocean to fake Gagarin's spaceflight too?
- You missed the Memo, both countries were controlled by Reptilians who live inside the hollow earth. Have you ever see Stalin and Reagan in the same room ? It's because it's the same person
  
  If the lizard people live in a hollow Earth, aren't they afraid to fall out of the bottom?

Mathematicians. On a sphere, you can make a triangle that consists of three 90 degree angles, adding up to 270. A flat triangle will require that all angles add up to 180, i.e. 60, 60, 60.
Edit: I'm not a mathematician, and yet I know this. So this one could probably include anyone who has ever had a math teacher that covered spherical geometry.
- I'm not sure that works though. For it to have all three angles at 90 degrees, wouldn't it need to cover a full 1/8th of the planet? Flat earthers would at lest understand that much. They would just deny anyone had measured that accurately. After all, anyone could go up in a spaceship to see the true shape of the earth, but only bigger governments and a few shitty billionaires have the means to do so.

Anybody who has ever flown far enough Eastwards or Westwards and saw how the time of day changed either much faster or slower than the time their clock said it was.
(Which can't be mathematically explained for a flat Earth but is a pretty simple effect to explain with a round Earth)
- Forget that. It's even simpler. Phone someone in a different time zone around sunset. If they say the sun isn't setting where they are, either the earth is a sphere, or they're lying and part of the conspiracy.
  
  If I understand correctly they have some kind of explanation where the sun works like a spotlight or something. But it also requires light to curve in weird ways to make any damn sense, soooo....

Functionally? It's gotta be well over a billion.

There are no conspiracists on the tail side, only on the heads side I am told, because they fell off

It's actually just me. I mind control everyone else. It's easier if I have some "nutjobs" as an opposition who know the real truth. And more fun for me.

10 % of the population ?
More or less... good estimate BTW.
In any case, a lot of people... way too many to cover up a lie that big.

Obviously only the programmer/team of programmers who wrote and maintains the simulation.
- The Earth is flat but they just make it look round with a shader.
  
  It's still flat because the entity observing the simulation is looking at an image on a monitor.