I'm optimistic and I'd say at least 2 orders of magnitude fewer.

It all started with some guys on youtube making probably quite decent money with their videos explaining that the earth is flat. Tons of people shared links to show others "look how stupid that guy must be". Because the videos were indeed made in the way to make you think the author is not very smart. I believe the suckers were those sharing the links and author decided that he'd rather have cash than his pride, outsmarting them all.

What surprised me, is that quite a few science channels made videos explaining and proving that the earth is not in fact flat, often providing links to some "flat-earther" video, basically helping him doing what he does.

Just being able to shoot a semi-decent video and uploading it to youtube, plus making some arguments for the earth being flat, takes way more comprehension than realizing that the earth is not flat. If there is some real flat-earther that made a video about it - you haven't seen it. Because it was likely low quality, not entertaining and he couldn't make it viral.

Not sure I agree, there are some pretty entertaining and well made movies about several religious topics. Also, one real flat earther that I know in person is a pretty good artist. He just has taken too many hallucinogens and gets drunk daily, which blunts his skeptic and logic abilities.