For new posts, it seems we have a hard coded limit of 200:
PHP:
$maxResults = 200; // TODO: customizable
...and a note that suggests we intended to make it customisable.
I think in XF1 it was customisable based on the maximumSearchResults option so we should either bring that back or make it customisable as it seems we intended.
For now, if you wanted to change it, you'd have to edit the code.
The above code is in the following file:
src/XF/ControllerPlugin/FindNew.php
and it'd just be a case of changing that value to the desired amount.
Or, if you wanted to base it on the actual max search results option (which I'm tempted to do for the next release) then you'd change it to:
PHP:
$maxResults = $this->options->maximumSearchResults;
I'm going to move this to bugs so we can consider changing it for the next release.