The review number is found at the top right of your interface.

This number indicates how many variations or positions are ready for you to review. A position is an individual arrangement of the chess pieces on the board as belonging to the course you are studying. If you have set your study preferences to review the "whole" variation, then while the review number may indicate "2 positions" to review, you may actually have to go through 2 whole variations instead (and therefore more than two positions!).

The review number changes depending on whether you are on the home page, where it shows all your cumulative positions to review across all courses, to showing just the course count or chapter count if you are browsing the course or inside a chapter of a course.