By default the gradebook shows the students in groups of 20 and it adds pagination elements in order to visualise all students’ qualifications.
If you wish to modify this number so that you can visualise more students (or all of them) you should:
1. click the Grades tab,
2. click on Setup and then on Preferences: Grader report,
3. in the General section set the number of students you would like to see in a page in “Students per page” (if you leave it in blank only 20 students will be shown per page by default):
4. save the changes.