You can do ctrl-w to find and enter - topclients, and that should take you to the line. add =25 or what ever number after Sources and Blocked.
example api.php?summaryRaw&getQuerySources=25&topClientsBlocked=25
then ctrl-o to write, then confirm name and press enter to save, then ctrl x to exit
It will take a bit, but should start to show you the larger list of clients.
on scripts/pi-hole/js/index.js we must add the amount on the parameters getQuerySources, topItems, topClients and topClientsBlocked.
Example, replace getQuerySources&topClientsBlocked for getQuerySources=50&topClientsBlocked=50
We really should have this as a WebUI setting! If no value is received as parameter - as it is on repo index.js -, api.php could query the value from settings instead of using default 10.