This sounds more realistic in terms of numbers. Thanks for the breakdown.
However, I do think your post represents a common misconception - if something is 0.5%, so 1 in 200, that is rare in terms of it not being very likely to happen to you personally, but in a huge group like all MN users, which is something like 8 million, it is still an enormous number of those users.
I don't think all of the 8 million users post of course. But even if only 1% of them post, that's still 800,000. And 0.5% of 800,000 is 4,000.
And of course it's not 1:1 where each narcissist will only interact closely with exactly 1 person. (Though, if you've grown up with an narcissistic parent or close relative, then it is supposed to be more likely you'll accept narcissistic behaviour from a friend or partner because it won't ring alarm bells or come across as weird like it would to someone who isn't familiar with it - so I think they do tend to cluster up around the same "victims"). But even using this very small model, if there are over 4,000 people who regularly post on MN who have experience with a narcissist, then they are likely to be attracted to thread titles which make them go "Wait a minute..." and reply to them.
What I mean is, even though something can be less-likely in terms of personal experience, it's not that unlikely that you'll find multiple people, in a large pool like active MN users, who have had that experience.
To compare, one of my children has a very minor birth defect which occurs in about 1 in 700 births. It's rare enough that the midwife delivering him had never seen it before, and I had never heard of it before he was born. When I made a post about it with the name of it in the title, I received several responses (I think about 5-10?) within a day or so, from posters whose children have the same thing. I don't think they were lying and I don't find the number implausible. The thread fell down the board after a fairly short time - if the subject matter had been something more interesting, the thread may have remained bumped for longer and I may have received even more responses. I think that's the effect you see with narcissism as a subject.
Yes I do think there is a bit of "Ooh that person is an arsehole - must be a narcissist!" and that does cloud things a bit. But there are also people who really truly do get how weird and discombobulating it can be to deal with someone who fits the description of narcissism.
If you do the Dunbar's Number thing, where we can "know" roughly 150 people at once, that means if narcissism is about 1 in 200, most people probably know a narcissist. But of course we don't know all of those 150 people equally, only a handful of them (15 seems to be the number given in research) would be close enough that something like narcissism would be apparent.
If it's all spread out totally equally, then roughly 3/4 of people likely know at least one narcissist even if they don't know that they are one, and about 10% of those (or 7.5% of all people, so 1 in 13-ish) will have the experience of knowing a narcissist closely enough to be able to post about it on MN.
Those are just extremely rough, back of envelope calculations. But 7.5% or 1 in 13 - seems about right to me? Especially if you include people who aren't aware that the relationship/relative/boss/colleague they encountered was a narcissist, or that narcissism even is a thing which has a name, but if you were to explain the crazy-making feelings of dealing with them, they would immediately recognise it.