The Problem
You have an old, established blog, and you left comment spam protection off (a tragic mistake). I did this recently with one of my personal blogs and in very little time I had 15,000 comments pending in my spam folder. I set about to delete the spam but found that I could only delete 20 at a time. How could I change the number of comments visible in the WordPress Admin ? I looked everywhere in the option and settings and could not find anywhere to have the wordpress admin display more. I even looked in the actual php files and could not find anything
The Cause
The option is actually hiding in plain sight.
The Solution
Click on “Screen Option” in the upper right” and in the box below “Show On Screen” and enter in however many you want to display.