<?php
if( have_posts() ) {
$ad_counter = 0;
$after_every = x //change this to the number of posts;
while( have_posts() ) {
$ad_counter++;
the_post();
?>
<h2><?php the_title(); ?></h2>
<?php
// Display content
$ad_counter = $ad_counter % $after_every;
if( 0 == $ad_counter ) {
echo '<h2 style="color:red;">Advertisement or custom content here</h2>';
}
}
}
?>