So does everyone get 1 reviewer each for their uploads or are multiple people reviewing them?
From what I understand, there are a number of reviewers for each marketplace (GraphicRiver, ThemeForest, etc.). Each day they go through a queue of submissions, generally only one reviewer will see a submission, though I think sometimes an item might require more review time and be passed onto another reviewer.
Imagine a row of office workers with a stack of files on their desks, they take a file from the top of the stack, look it over, make sure it meets the technical and visual requirements, then either stamp it as approved or rejected, then they move onto the next one. All the while more files are constantly being added to the bottom of the stack.