How can I view the number of times an attendance code has been used, or how many times an individual student has it assigned to them?

If you go to Reporting > Reports > Attendance (tab) > Code Count you are able to view the amount of a specific code a student has assigned to them over a given date range. 

The Code Count by Group Report enables you to report on how many times each code has been used by year group and form, and you can toggle between count and percentages.