<?php
//Insert ads after second paragraph of single post content.
add_filter( 'the_content', 'bs_insert_post_ads' );
function bs_insert_post_ads( $content ) {
    $ad_code = '<div>Ads code here</div>';
    $paragraph_no_to_insert = 2;
    if ( is_single() && ! is_admin() ) {
        return bs_insert_after_paragraph( $ad_code, $paragraph_no_to_insert, $content );
    }
    return $content;
}

// Parent Function that makes the magic happen
function bs_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 );
}