Do you base the number of reviews a story has (yours or someone else's) on how good or bad it must be?
Nope. I don't pay much attention to how many reviews other people's stories have, and as far as my own go, I can usually understand why they got the number of reviews they did. I know that my Nick stories are generally going to get more reviews per chapter than my stories about the other guys because Nick just tends to attract a larger audience. Along the same lines, an AU is usually not going to get as many reviews as a non-AU.
Longer stories mean more reviews, as long as the chapters are posted one at a time. Post more than one chapter at a time, and some people will only review once for the whole update. That's why, of my older stories that I put on AC after they had been finished on my site for awhile, the ones that I posted all at one time only have a few reviews, while the ones that I posted a few chapters at a time have more. Broken was really popular on my site, but doesn't even average one review per chapter on AC because I didn't post it here until it had been finished on my site for over a year, so most people had already read it there. By My Side was the same way; I always posted chapters on my site before I added them to AC, so most people who were following it read them there first and emailed me with feedback instead of leaving a review.
Like Mare, I always hope to average a few reviews per chapter, but I know that for some stories it's going to be slightly more, and some it's going to be less, and that's just how it is. I don't go, "Wow, this story must suck!" if it gets less than 3 reviews per chapter... unless it really does suck LOL.