Inject Ads To WordPress Comments - See https://wordpress.stackexchange.com/a/328155/26350

functions.php

/**
 * Inject Ads To WordPress Comments - end-callback for wp_list_comments().
 *
 * The Ads HTML is sanitized through wp_kses_post().
 *
 * @version 1.0.11
 * @see https://wordpress.stackexchange.com/a/328155/26350
 */
function wpse_comments_ads_injection( $comment, $args, $depth ) {
	static $instance = 0;
	$tag             = ( 'div' == $args['style'] ) ? 'div' : 'li';
	$ads_rate        = isset( $args['_ads_rate' ] ) && $args['_ads_rate' ] > 0 
		? (int) $args['_ads_rate' ] 
		: 2;
	$ads_offset      = isset( $args['_ads_offset' ] ) && $args['_ads_offset' ] > 0
    		? (int) $args['_ads_offset' ]
		: 0;
	$ads_html        = isset( $args['_ads_html' ] ) 
		? wp_kses_post( $args['_ads_html' ] )
		: '';
	
	echo "</{$tag}><!-- #comment-## -->\n";

         if ( $ads_offset <= $instance && 0 === ( $instance - $ads_offset ) % $ads_rate && ! empty( $ads_html ) ) {
		echo "<{$tag} class=\"comment__ad\">{$ads_html}</{$tag}>";
	}

	$instance++;
}

/**
 * Filter wp_comments_list().
 *
 * @see https://wordpress.stackexchange.com/a/328155/26350
 */                                                                  
add_filter( 'wp_list_comments_args', function( $args ) {
	if ( ! isset( $args['end-callback'] ) ) {
		// Modify to your needs:
		$args['end-callback'] = 'wpse_comments_ads_injection';
		$args['_ads_offset']  = 0;  // Start injecting after the 1st comment.
		$args['_ads_rate']    = 2;  // Inject after every second comment.
		$args['_ads_html']    = '<img src="http://placekitten.com/g/500/100" />'; // HTML for the AD.
	}
    return $args;
} );