Display related post’s title within single post’s texts in same category. Actually insert the related post title after each 12, 17 and 25 paragraphs.

functions.php

function my_post_list($title = 'Read More:', $args = array()){
global $post; 
$project_category = wp_get_post_categories($post->ID);
$defaults = array(  // Set some defaults for querying the Posts
	'ignore_sticky_posts'   => true,
        'posts_per_page'        => 1,
        'post_status'           => 'publish',
        'category__in' => $project_category,
        'orderby'               => 'rand'
);
// Parse any arguments passed to this function with the defaults to create a final '$args' array
$args = wp_parse_args($args, $defaults);
$random = new WP_Query($args);  // Query the Posts
    if($random->have_posts()) : // Check to make sure some random Posts were returned
	echo '<div class="read-more">';
        echo '<h4>' . $title . '</h4>';
        echo '<ul>';
        // Create a new custom Loop, and for each Post set up the postdata
	while($random->have_posts()) : $random->the_post(); 
            printf('<li id="random-post-%1$s">', get_the_ID());
            printf(
                '<a href="%1$s" title="%2$s">%3$s</a>',
                get_permalink(get_the_ID()),
                the_title_attribute('before=Check out \'&after=\'&echo=0'),
                get_the_title()
            );
            echo '</li>';
        endwhile;
        wp_reset_postdata();    // Reset the postdata so that the rest of your Loop will work correctly
        echo '</ul>';
	echo '</div>';	
  endif;
}

single.php

//the_content(); No need to use the default the_content() function
$content = apply_filters('the_content', get_the_content());
$content = explode("</p>", $content);
	    				
for ($i = 0; $i <count($content); $i++) {
	if ($i == 12) {
	my_post_list();
	}
	if ($i == 17) {
	my_post_list();
	}
	if ($i == 25) {
	my_post_list();
	}
echo $content[$i] . "</p>";
}