You'd do well to take a smaller selection of distributions to focus on, because
1) you can't possibly list all existing distributions, because very probably they're not all listed in one place (or even in few enough places for you to go through)
2) new distributions probably get released during the time you write your work
3) even listing the names would be a massive piece of work, and giving any further details would probably be just too much
4) the reader (whoever that is) would get more out of your work, as would you, if you paid more attention to a smaller number of distributions.
Even if there existed 500+ distributions, a lot of them were very similar to each other, perhaps differing only in preinstalled package selection (or their versions), themes or such. If you instead focus on distributions that differ notably from each other, and are used by more than only few people in addition to the creator, you'll get somewhere. For example you could start off by studying the differences between RedHat (the commercial ones), Slackware, Gentoo and Debian. You could take in some notable similar distributions, like Ubuntu or Fedora, and see how and why they are separate projects. Then you could eye some "specialized" distributions, that are aimed at gaming, being "Windows-like" and so on, and you'd soon have plenty enough names to write on. These are just some ideas to ease your work, because if you'll take too big a bite, you'll get bored and quit the work before it's finished--and that's not what you're after, right?
Remember to mention the date when you checked the list you're referring to, because web content changes all the time, and the distribution list you read probably isn't the same anymore when the reader gets to it.