Hi Ritchie, this was an interesting query, and yes you're correct, it's currently hard-coded to 5 in the php code.
Generally, it's really not a good idea to alter the core moodle php code, but if this is important, the place to edit it is here:
Line 3736 (This could well be different, based on moodle version, this is from 3.5.4+ (Build: 20190301))
This example snippet is my git diff showing that I've altered the original value from 5 to 4, for testing...
I wasn't able to find a relevant tracker ticket about making this specific page limit configurable, which would be the more ideal solution to be developed long-term.