Derken çözümü buldum

$my_query = new WP_Query( array( 'cat' => 'catID', 'showposts' => '20', 'paged' => get_query_var('paged') ) );
while ( $my_query->have_posts() ) : $my_query->the_post();
the_title();
// more stuff here
endwhile;
wp_pagenavi( array( 'query' => $my_query ) );
wp_reset_postdata(); // avoid errors further down the pageİyi çalışmalar.