As far as I know there is no way to show more than the default amount of PMs per page like you can with forum topics and posts.
To acquire more PM slots you can become a customer by purchasing FG and furthermore a verified member to unlock additional PM space (and other cool benefits) - read more about that here:
https://forums.d2jsp.org/topic.php?t=74114203&f=28<-- you can see the verified icon next to my group name, for example, and I have 150 PM slots
Also just a heads up this is the site suggestion area, you can post general questions about d2jsp or anything else here:
https://forums.d2jsp.org/forum.php?f=34This post was edited by Epiphany on Aug 11 2020 10:00pm