2 choices here, you either need to set the category as an array e.g.
$args = array(
'posts_per_page' => 5,
'category' => array(15,16,17,18,19)
);
You can’t just add the numbers in a list but I can’t find any documentation that the category element allows multiples (as the name is category)
The other option is to use wp_query and category__in
$query = new WP_Query(
array(
'category__in' => array(15,16,17,18,19),
'posts_per_page' => 5,
'post_type' => 'post',
)
);
if ( $query->have_posts() ) {
while ( $query->have_posts() ) {
$query->the_post();
// do something
}
}
wp_reset_postdata();
These will get the 5 last posts from any of those categories, if you want to get a post from each of them then the query needs to be a lot different