The fact that an opinion has been widely held is no evidence that it is not utterly absurd; indeed, in view of the silliness of the majority of mankind, a widespread belief is more likely to be foolish than sensible. – Bertrand Russell.
The thing to keep in mind here is that the truth or falsity of a claim depends on the evidence that supports it (or perhaps refutes it), and the number of people who believe a particular thing is totally irrelevant. Millions of people believe they have seen UFOs; millions more think they have seen a ghost; millions more are certain that psychics can put them in contact with the dead. The only thing lacking in all of those scenarios is confirmable evidence.
I’ve come across some self-styled paranormal “experts” who use this kind of fallacious reasoning regularly. Their argument goes like this: “A large number of people – maybe hundreds – claim to have seen a particular UFO phenomenon at the same time. Therefore we should accept the reality of alien visitation.”
A similar argument goes like this: “Thousands of people have reported individual, lone encounters with cryptids [Bigfoot and other non-existent creatures]. Therefore, the sheer volume of reports means that we should accept the existence of these alleged animals.”
What all of these accounts of paranormal reports have in common, of course, is a singular lack of evidence other than the unsupported claims that are made – however sincere those claims might be. And promoters of paranormal piffle tend to compound matters by saying such inane things as, “Could so many people be mistaken?” (Well, yes, actually.)
Keep in mind the fact that the number of people who believe something is entirely independent of whether that “something” is true or not. Try this idea:
Christianity is the true religion because there are over two billion people who believe in it.
Now if we were able to ask the seven billion people on earth this question: is Christianity the true religion? then there would certainly be five billion people who say “no.” For them, Christianity is a false religion.
Try it again with Islam: for about 5.4 billion others, it, too, is a false religion.
Then try it with Judaism, Hinduism, Sikhism, etc. What soon becomes clear is that if the argument to popularity is reliable, then 100% of the people on Earth agree that the other fellow’s religion is false, so all religions must be false.
Personally, I do think that all religions are false, but the argument to popularity doesn’t prove it, in the same way it doesn’t prove that any religion is true, or that aliens are here, or that cryptids exist or that ghosts, psychics and all the rest of it are real. The argument to popularity is one of the weakest arguments it is possible to use when trying to prove a point, but it does seem to be quite popular, as it were – at least among those who have absolutely nothing else to support their extraordinary claims.
The bottom line.
The argument to popularity (argumentum ad populum) is a fallacy of irrelevance. The number of people who believe a particular proposition does not determine whether it is true or not. A proposition stands or falls on the evidence that supports it, no matter how many people believe it (or disbelieve it, for that matter).