I would say that most books only have one or two POV characters. Having more than that can get confusing, and the writer’s job is to clarify, not to confuse. It also makes the writer’s job easier to have only a few POV characters… writing is hard, and why would you want to make it harder than it is?
The way I see it, main characters and POV characters are different. A main character is a character that appears often in the story (but you’re not necessarily getting into his POV), while we get to actually see the world from a POV character’s eyes. I’d say it’s OK to have more than two main characters – and a story doesn’t quite work if you only have one main character, unless he’s got a split personality disorder or living in a dreamworld, and both of those scenarios would be hard to pull off in fiction.
With most of my stories, I find that I usually have around five main characters and one or two POV characters. Most of the time, I don’t consciously pick five main characters, it just so happens that when I’m doing the outline, five main characters is what I need to accomplish the story’s goals. Kind of strange.
I suppose the number of main and POV characters would depend on the length of the work. Obviously, shorter novels are going to need fewer, and long fantasy or classic novels will need more. But I would go with whatever is least likely to confuse the reader, and whatever is least likely to drive you crazy as a writer.