What's the number next the "review answer/question" button?
Is it the number of people who have reviewed the question/answer?
It's the quality of the post; the number of users who already reviewed it is not shown.
The quality number can be a negative or a positive number; the highest number I have seen is 79 (in the "low quality posts" tab for the review page), and the lowest is −7. As negative numbers are used, I think that the higher the number is, the higher is the post's quality.