Hi Wendy
This report looks as though it is not a report manager report so it is not able to be modified from the user interface.
The report code is in mod/certificate/report.php so would have to be modified by a developer . There is a variable CERT_MAX_PER_PAGE that could be adjusted,
I had a look at the report for Totara 13 and this looks different but not sure if this displays more users . What version are you on?
Regards