A standard way to change the number of search results displayed is something like this:
$myquery = "&posts_per_page=5";
$myquery = $query_string.$myquery;
query_posts($myquery);
Unfortunately, this breaks Relevanssi. Relevanssi hasn’t played nice with posts_per_page in any case, for some reasons I don’t really understand — this is legacy code from the original wpSearch plugin.
However, from Premium version 1.4.4 and free 2.9.2, you can now adjust the search results per page in a fairly easy and clean manner. Add this to your functions.php file:
add_filter('post_limits', 'postsperpage');
function postsperpage($limits) {
if (is_search()) {
global $wp_query;
$wp_query->query_vars['posts_per_page'] = 10;
}
return $limits;
}
Adjust the value (10) the way you want.