Go for two.
I noticed on fanfiction, where you can actually see how many hits you get per chapter, the farther a reader goes into a story, the fewer hits you get. So, it would follow, using that logic, that the more chapters you have, the fewer hits to the end.
A couple of other factors, to my thinking are these:
A lot of people come to the site and read whatever random story that comes up. Nine out of 10 times, it'll probably be a puerile one.
Another reason that people visit the site is that they know someone who is a writer here and will look them up.
So, to sum up: short attention span.
Motive is selfish
short attention span.
I made up a really long fantasy, which I enjoyed emensely, so I didn't care enough to ask the question that you are so thoughtfully posing. I think the longest a story might be on this format, is probably 18 chapters. (Depending on the length of the chapter of course, I figure 4 - 6 pages each).
That's my opinion. Maybe I'm wrong, but that seems to be the way of it. Just ask yourself, when you read a story and you click on the drop-down chapter menu, do you leave if you see it more than 10 chapters?