Often, I have a button next to review with a number on it. This number doesn't correlate to the number of posts available for me to review, as I can click on it and find no reviews available. What does this number mean?
It is just as Torisuda says, just a indicator of the amount of items that are ready for review. This number tends to be a bit inacurate at times ;)
This indicator is shown only to moderators and users who've gained access to the moderator tools. As such, it behaves in keeping with the design of those tools:
If you poke around in /tools, you'll quickly notice that most of what's found there isn't directly actionable - it's informational! What you do with that information is up to you: hopefully, you've been around long enough now that you have some idea of how you want the site to be run, and are able to make good use of the information presented.
As quoted from shog9's answer on why the number is showing a higher amount then the amount of reviews available