The very early version of the plugin tried to dynamically read the change and file data from Perforce, but this was full of problems; it was slow and often the connection details would be out of scope. For the last 3 years or so the changes and files for each change are stored in jobs/JOBNAME/builds/NNN/changelog.xml, however there are links to Swarm if you use it as your Review tool/browser.
If "Maximum number of changes shown in a build" is 0 or negative, then it is set to DEFAULT_CHANGE_LIMIT.
DEFAULT_FILE_LIMIT = 50
DEFAULT_CHANGE_LIMIT = 20
The Global Configuration uses the same defaults, but only works for new Jenkins installs. I don't reset the defaults if a user has previously set a value.