gistfile1.php

<?php
// Example 1
$show_after_p = 2;
$content = apply_filters('the_content', $post->post_content);
if(substr_count($content, '<p>') > $show_after_p)
{
	$contents = explode("</p>", $content);
	$p_count = 1;
	foreach($contents as $content)
	{
		echo $content;

		if($p_count == $show_after_p)
		{
		?>
                YOUR AD CODE GOES HERE
		<?php
		}
		echo "</p>";
		$p_count++;
	}
}

//Example 2

//Insert ads after second paragraph of single post content.
add_filter( 'the_content', 'prefix_insert_post_ads' );
function prefix_insert_post_ads( $content ) {
	$ad_code = '<div>Ads code goes here</div>';
	if ( is_single() && ! is_admin() ) {
		return prefix_insert_after_paragraph( $ad_code, 2, $content );
	}
	return $content;
}
// Parent Function that makes the magic happen
function prefix_insert_after_paragraph( $insertion, $paragraph_id, $content ) {
	$closing_p = '</p>';
	$paragraphs = explode( $closing_p, $content );
	foreach ($paragraphs as $index => $paragraph) {
		if ( trim( $paragraph ) ) {
			$paragraphs[$index] .= $closing_p;
		}
		if ( $paragraph_id == $index + 1 ) {
			$paragraphs[$index] .= $insertion;
		}
	}
	return implode( '', $paragraphs );
}